Add abstract method execute() to \Magento\Framework\App\Action\Action

Purpose
==

Make implementing Actions more developer friendly via self-documenting code.

Background
==

The existing method `\Magento\Framework\App\Action\Action::dispatch()` calls
an `execute()` method without actually implementing it.

```php
$result = $this->execute();
```

Any subclass actions simply need to implement that method and it will be
called automatically when the superclass method is `dispatch()` is called.

Without the changes in this PR, the `execute()` method in effect was part
of an implicit interface exposed by the `Action` class.

Because of this the `Action` class could never be instantiated and used (that
is, dispatched), without first subclassing it.
This in turn makes the `Action` class an implicitly abstract class.

All this PR does it that it makes the class interface explicit by adding the
execute method as an abstract method to `\Magento\Framework\App\Action\Action`.

All the other changes in this PR are just follow-up changes to accommodate
for this one update.

Follow up changes
--

Because concrete classes can't have abstract methods, the `Action` class
itself also had to be made abstract.

A number of subclasses of `Action` that did not implement `execute()`,
because they themselves also needed to be extended, had to be declared as
abstract classes now, too, because they inherit the abstract method from
the `Action` superclass.

There where some tests that instantiated these implicitly abstract classes.
This PR adds stub implementations for each of these.

* `Magento\Checkout\Test\Unit\Controller\OnepageTest`
  in app/code/Magento/Checkout/Test/Unit/Controller/
* `Magento\Contact\Test\Unit\Controller\IndexTest`
  in app/code/Magento/Contact/Test/Unit/Controller/
* `Magento\Tax\Test\Unit\App\Action\ContextPluginTest`
  in app/code/Magento/Tax/Test/Unit/App/Action/
* `Magento\Catalog\Helper\Product\ViewTest`
  in dev/tests/integration/testsuite/Magento/Catalog/Helper/Product/
* `Magento\Cms\Helper\PageTest`
  in dev/tests/integration/testsuite/Magento/Cms/Helper/Stub/
* `Magento\Sales\Controller\Adminhtml\Order\CreateTest`
  in dev/tests/integration/testsuite/Magento/Sales/Controller/Adminhtml/Order/

Finally, the `Magento\Ui\Controller\UiActionInterface` class, which was
implemented by an abstract subclass of `Action`, namely
`\Magento\Ui\Controller\Adminhtml\AbstractAction`, contained a
`public function execute();` itself.
This explicit interface contract was in conflict with the implicit interface
of the `Action` class, which did defined the same method, but implicitly.

To resolve this conflict this PR changes the method exposed by the
`UiActionInterface` to `public function executeAjaxRequest();`. This
seemed the most fitting when looking at the code implementing the interface
method. Internally it only calls the now explicit `execute()` method inherited
from the `Action` interface.

Also, it seems as if the `UiActionInterface::execute()` method was only called
by a single test. However, I realize the method might be used a lot more in the
non-public EE code, which will also need to be adjusted to be compatible with
this PR.

Reasons for this PR
==

As stated above, the purpose of this PR is to make the developer experience
better. This is done by making an implicit interface explicit. This has
a whole number of benefits, besides being good OOP practice:

* It is clear which classes should be instantiated and which should not
* It is clear which method needs to be implemented when subclassing `Action`
* The expected return type of `execute()` is documented in the phpdoc block
* When subclassing `Action` the IDE can be used to quickly a method stub

Allover the changes lead to more self-documenting code that tells developers
to they should use it.

Potential issues
==

The explicit abstract `execute()` method added in this PR is declared with
protected visibility.
This basically removes it as a valid extension point for plugins. However,
since the method was not declared as public previously (in fact, is wasn't
declared at all), it should not have been used as an extension point for
plugins anyway.

So the main reason against this PR I can think of is that the EE code will
need to be adjusted (plus, I might be missing many obvious reasons).
Of course I'd be happy to make the required updates, but I realize that is
problematic due to the commercial licenced, non-open code.
I've got a bunch of hacky shell scripts to find and update any controllers
needed to be declared abstract, but I'd rather not share them as they are just
one-time hacks.
All the other changes - updating the ui action interface and the test stubs -
where done manually and didn't take long.

app/code/Magento/AdminNotification/Controller/Adminhtml/Notification.php

@@ -7,7 +7,7 @@
  */
 namespace Magento\AdminNotification\Controller\Adminhtml;
 
-class Notification extends \Magento\Backend\App\AbstractAction
+abstract class Notification extends \Magento\Backend\App\AbstractAction
 {
     /**
      * @return bool

app/code/Magento/Backend/App/Action.php

@@ -12,6 +12,6 @@
 /**
  * @SuppressWarnings(PHPMD.NumberOfChildren)
  */
-class Action extends \Magento\Backend\App\AbstractAction
+abstract class Action extends \Magento\Backend\App\AbstractAction
 {
 }

app/code/Magento/Backend/Controller/Adminhtml/Auth.php

@@ -10,7 +10,7 @@
 /**
  * Auth backend controller
  */
-class Auth extends AbstractAction
+abstract class Auth extends AbstractAction
 {
     /**
      * Check if user has permissions to access this controller

app/code/Magento/Backend/Controller/Adminhtml/Cache.php

@@ -8,7 +8,7 @@
 use Magento\Backend\App\Action;
 use Magento\Framework\Exception\LocalizedException;
 
-class Cache extends Action
+abstract class Cache extends Action
 {
     /**
      * @var \Magento\Framework\App\Cache\TypeListInterface

app/code/Magento/Backend/Controller/Adminhtml/Dashboard.php

@@ -11,7 +11,7 @@
  */
 namespace Magento\Backend\Controller\Adminhtml;
 
-class Dashboard extends \Magento\Backend\App\Action
+abstract class Dashboard extends \Magento\Backend\App\Action
 {
     /**
      * @return bool

app/code/Magento/Backend/Controller/Adminhtml/Index.php

@@ -10,7 +10,7 @@
 /**
  * Index backend controller
  */
-class Index extends AbstractAction
+abstract class Index extends AbstractAction
 {
     /**
      * Check if user has permissions to access this controller

app/code/Magento/Backend/Controller/Adminhtml/System.php

@@ -12,7 +12,7 @@
  *
  * @author     Magento Core Team <core@magentocommerce.com>
  */
-class System extends AbstractAction
+abstract class System extends AbstractAction
 {
     /**
      * @return bool

app/code/Magento/Backend/Controller/Adminhtml/System/Account.php

@@ -12,7 +12,7 @@
  *
  * @author      Magento Core Team <core@magentocommerce.com>
  */
-class Account extends Action
+abstract class Account extends Action
 {
     /**
      * @return bool

app/code/Magento/Backend/Controller/Adminhtml/System/Design.php

@@ -7,7 +7,7 @@
 
 use Magento\Backend\App\Action;
 
-class Design extends Action
+abstract class Design extends Action
 {
     /**
      * Core registry

app/code/Magento/Backend/Controller/Adminhtml/System/Store.php

@@ -17,7 +17,7 @@
  *
  * @author      Magento Core Team <core@magentocommerce.com>
  */
-class Store extends Action
+abstract class Store extends Action
 {
     /**
      * Core registry

app/code/Magento/Backup/Controller/Adminhtml/Index.php

@@ -10,7 +10,7 @@
  *
  * @author      Magento Core Team <core@magentocommerce.com>
  */
-class Index extends \Magento\Backend\App\Action
+abstract class Index extends \Magento\Backend\App\Action
 {
     /**
      * Core registry

app/code/Magento/Catalog/Controller/Adminhtml/Category.php

@@ -8,7 +8,7 @@
 /**
  * Catalog category controller
  */
-class Category extends \Magento\Backend\App\Action
+abstract class Category extends \Magento\Backend\App\Action
 {
     /**
      * Initialize requested category and put it into registry.

app/code/Magento/Catalog/Controller/Adminhtml/Category/Widget.php

@@ -12,7 +12,7 @@
  *
  * @author     Magento Core Team <core@magentocommerce.com>
  */
-class Widget extends \Magento\Backend\App\Action
+abstract class Widget extends \Magento\Backend\App\Action
 {
     /**
      * @var \Magento\Framework\View\LayoutFactory

app/code/Magento/Catalog/Controller/Adminhtml/Product.php

@@ -11,7 +11,7 @@
  * Catalog product controller
  * @SuppressWarnings(PHPMD.NumberOfChildren)
  */
-class Product extends \Magento\Backend\App\Action
+abstract class Product extends \Magento\Backend\App\Action
 {
     /**
      * @var Product\Builder

app/code/Magento/Catalog/Controller/Adminhtml/Product/AbstractProductGrid.php

@@ -10,7 +10,7 @@
 use Magento\Catalog\Controller\Adminhtml\Product;
 use Magento\Framework\Controller\Result;
 
-class AbstractProductGrid extends Product
+abstract class AbstractProductGrid extends Product
 {
     /**
      * @var \Magento\Framework\View\Result\LayoutFactory

app/code/Magento/Catalog/Controller/Adminhtml/Product/Action/Attribute.php

@@ -13,7 +13,7 @@
 /**
  * Adminhtml catalog product action attribute update controller
  */
-class Attribute extends Action
+abstract class Attribute extends Action
 {
     /**
      *  @var \Magento\Catalog\Helper\Product\Edit\Action\Attribute

app/code/Magento/Catalog/Controller/Adminhtml/Product/Attribute.php

@@ -12,7 +12,7 @@
 use Magento\Framework\Controller\Result;
 use Magento\Framework\View\Result\PageFactory;
 
-class Attribute extends \Magento\Backend\App\Action
+abstract class Attribute extends \Magento\Backend\App\Action
 {
     /**
      * @var \Magento\Framework\Cache\FrontendInterface

app/code/Magento/Catalog/Controller/Adminhtml/Product/Set.php

@@ -10,7 +10,7 @@
  *
  * @author      Magento Core Team <core@magentocommerce.com>
  */
-class Set extends \Magento\Backend\App\Action
+abstract class Set extends \Magento\Backend\App\Action
 {
     /**
      * Core registry

app/code/Magento/Catalog/Controller/Product.php

@@ -10,7 +10,7 @@
 use Magento\Catalog\Controller\Product\View\ViewInterface;
 use Magento\Catalog\Model\Product as ModelProduct;
 
-class Product extends \Magento\Framework\App\Action\Action implements ViewInterface
+abstract class Product extends \Magento\Framework\App\Action\Action implements ViewInterface
 {
     /**
      * Initialize requested product object

app/code/Magento/Catalog/Controller/Product/Compare.php

@@ -15,7 +15,7 @@
  * @SuppressWarnings(PHPMD.LongVariable)
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
-class Compare extends \Magento\Framework\App\Action\Action
+abstract class Compare extends \Magento\Framework\App\Action\Action
 {
     /**
      * Customer id

app/code/Magento/CatalogRule/Controller/Adminhtml/Promo/Catalog.php

@@ -18,7 +18,7 @@
 use Magento\Framework\Registry;
 use Magento\Framework\Stdlib\DateTime\Filter\Date;
 
-class Catalog extends Action
+abstract class Catalog extends Action
 {
     /**
      * Dirty rules notice message

app/code/Magento/CatalogRule/Controller/Adminhtml/Promo/Widget.php

@@ -7,7 +7,7 @@
 
 use Magento\Backend\App\Action;
 
-class Widget extends Action
+abstract class Widget extends Action
 {
     /**
      * @return bool

app/code/Magento/CatalogWidget/Controller/Adminhtml/Product/Widget.php

@@ -10,7 +10,7 @@
 /**
  * Class Widget
  */
-class Widget extends Action
+abstract class Widget extends Action
 {
     /**
      * @return bool

app/code/Magento/Checkout/Controller/Cart.php

@@ -12,7 +12,7 @@
 /**
  * Shopping cart controller
  */
-class Cart extends \Magento\Framework\App\Action\Action implements ViewInterface
+abstract class Cart extends \Magento\Framework\App\Action\Action implements ViewInterface
 {
     /**
      * @var \Magento\Framework\App\Config\ScopeConfigInterface

app/code/Magento/Checkout/Controller/Onepage.php

@@ -13,7 +13,7 @@
 /**
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
-class Onepage extends Action
+abstract class Onepage extends Action
 {
     /**
      * @var array

app/code/Magento/Checkout/Test/Unit/Controller/OnepageTest.php

@@ -85,9 +85,9 @@ protected function setUp()
         $context->expects($this->once())
             ->method('getEventManager')
             ->willReturn($this->eventManager);
-
+        
         $this->controller = $objectManager->getObject(
-            'Magento\Checkout\Controller\Onepage',
+            'Magento\Checkout\Test\Unit\Controller\Stub\OnepageStub',
             [
                 'context' => $context
             ]

app/code/Magento/Checkout/Test/Unit/Controller/Stub/OnepageStub.php

@@ -0,0 +1,12 @@
+<?php
+
+
+namespace Magento\Checkout\Test\Unit\Controller\Stub;
+
+class OnepageStub extends \Magento\Checkout\Controller\Onepage
+{
+    protected function execute()
+    {
+        // Empty method stub for test
+    }
+}

app/code/Magento/CheckoutAgreements/Controller/Adminhtml/Agreement.php

@@ -5,7 +5,7 @@
  */
 namespace Magento\CheckoutAgreements\Controller\Adminhtml;
 
-class Agreement extends \Magento\Backend\App\Action
+abstract class Agreement extends \Magento\Backend\App\Action
 {
     /**
      * Core registry

app/code/Magento/Cms/Controller/Adminhtml/Block.php

@@ -10,7 +10,7 @@
  *
  * @author      Magento Core Team <core@magentocommerce.com>
  */
-class Block extends \Magento\Backend\App\Action
+abstract class Block extends \Magento\Backend\App\Action
 {
     /**
      * Core registry

app/code/Magento/Cms/Controller/Adminhtml/Wysiwyg/Images.php

@@ -10,7 +10,7 @@
  *
  * @author      Magento Core Team <core@magentocommerce.com>
  */
-class Images extends \Magento\Backend\App\Action
+abstract class Images extends \Magento\Backend\App\Action
 {
     /**
      * Core registry

app/code/Magento/Contact/Controller/Index.php

@@ -12,7 +12,7 @@
 /**
  * Contact index controller
  */
-class Index extends \Magento\Framework\App\Action\Action
+abstract class Index extends \Magento\Framework\App\Action\Action
 {
     /**
      * Recipient email config path

app/code/Magento/Contact/Test/Unit/Controller/IndexTest.php

@@ -51,7 +51,7 @@ public function setUp()
             )
         );
 
-        $this->_controller = new \Magento\Contact\Controller\Index(
+        $this->_controller = new \Magento\Contact\Test\Unit\Controller\Stub\IndexStub(
             $context,
             $this->getMock('\Magento\Framework\Mail\Template\TransportBuilder', [], [], '', false),
             $this->getMockForAbstractClass('\Magento\Framework\Translate\Inline\StateInterface', [], '', false),

app/code/Magento/Contact/Test/Unit/Controller/Stub/IndexStub.php

@@ -0,0 +1,12 @@
+<?php
+
+
+namespace Magento\Contact\Test\Unit\Controller\Stub;
+
+class IndexStub extends \Magento\Contact\Controller\Index
+{
+    protected function execute()
+    {
+        // Empty method stub for test
+    }
+}

app/code/Magento/CurrencySymbol/Controller/Adminhtml/System/Currency.php

@@ -11,7 +11,7 @@
  */
 namespace Magento\CurrencySymbol\Controller\Adminhtml\System;
 
-class Currency extends \Magento\Backend\App\Action
+abstract class Currency extends \Magento\Backend\App\Action
 {
     /**
      * Core registry

app/code/Magento/CurrencySymbol/Controller/Adminhtml/System/Currencysymbol.php

@@ -11,7 +11,7 @@
  */
 namespace Magento\CurrencySymbol\Controller\Adminhtml\System;
 
-class Currencysymbol extends \Magento\Backend\App\Action
+abstract class Currencysymbol extends \Magento\Backend\App\Action
 {
     /**
      * Check the permission to run it

app/code/Magento/Customer/Controller/Account.php

@@ -16,7 +16,7 @@
  * @SuppressWarnings(PHPMD.NumberOfChildren)
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
-class Account extends \Magento\Framework\App\Action\Action
+abstract class Account extends \Magento\Framework\App\Action\Action
 {
     /**
      * List of actions that are allowed for not authorized users

app/code/Magento/Customer/Controller/Address.php

@@ -12,7 +12,7 @@
  *
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
-class Address extends \Magento\Framework\App\Action\Action
+abstract class Address extends \Magento\Framework\App\Action\Action
 {
     /**
      * @var \Magento\Customer\Model\Session

app/code/Magento/Customer/Controller/Adminhtml/Cart/Product/Composite/Cart.php

@@ -13,7 +13,7 @@
  *
  * @author      Magento Core Team <core@magentocommerce.com>
  */
-class Cart extends \Magento\Backend\App\Action
+abstract class Cart extends \Magento\Backend\App\Action
 {
     /**
      * Customer we're working with

app/code/Magento/Customer/Controller/Adminhtml/Group.php

@@ -11,7 +11,7 @@
 /**
  * Customer groups controller
  */
-class Group extends \Magento\Backend\App\Action
+abstract class Group extends \Magento\Backend\App\Action
 {
     /**
      * Core registry

app/code/Magento/Customer/Controller/Adminhtml/Index.php

@@ -24,7 +24,7 @@
  * @SuppressWarnings(PHPMD.TooManyFields)
  * @SuppressWarnings(PHPMD.NumberOfChildren)
  */
-class Index extends \Magento\Backend\App\Action
+abstract class Index extends \Magento\Backend\App\Action
 {
     /**
      * @var \Magento\Framework\Validator

app/code/Magento/Customer/Controller/Adminhtml/System/Config/Validatevat.php

@@ -10,7 +10,7 @@
  *
  * @author     Magento Core Team <core@magentocommerce.com>
  */
-class Validatevat extends \Magento\Backend\App\Action
+abstract class Validatevat extends \Magento\Backend\App\Action
 {
     /**
      * Perform customer VAT ID validation

app/code/Magento/Customer/Controller/Adminhtml/Wishlist/Product/Composite/Wishlist.php

@@ -10,7 +10,7 @@
 /**
  * Catalog composite product configuration controller
  */
-class Wishlist extends \Magento\Backend\App\Action
+abstract class Wishlist extends \Magento\Backend\App\Action
 {
     /**
      * Wishlist we're working with.

app/code/Magento/DesignEditor/Controller/Adminhtml/System/Design/Editor.php

@@ -14,7 +14,7 @@
  *
  * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
  */
-class Editor extends \Magento\Backend\App\Action
+abstract class Editor extends \Magento\Backend\App\Action
 {
     /**
      * @var \Magento\Theme\Model\Config