Add a new event 'adminhtml_widget_tabs_add_after' for adding custom tab.

[kiatng]

There are only a handful for events in core block and none is useful for adding custom tab in admin pages.

1. core_block_abstract_prepare_layout_after can be used to add custom tab, but it doesn't set the active tab correctly.
2. adminhtml_block_html_before would not render the custom tab at all as we need html_after.
3. core_block_abstract_to_html_after also wouldn't render the custom tab.

So the reliable option is to overwrite Mage_Adminhtml_Block_Customer_Edit_Tabs, which is what happened with some 3rd-party extensions, totally not ideal.

Here's the observer for reference when the event adminhtml_widget_tabs_add_after is added:

    public function addTab($observer)
    {
        /**
         * @var Mage_Adminhtml_Block_Customer_Edit_Tabs $block block data:
         * [id] => customer_info_tabs
         * [title] => Customer Information
         * [type] => adminhtml/customer_edit_tabs
         * [module_name] => Webkul_Marketplace // 3rd-party extension overwrite
         */
        $block = $observer->getBlock();
        //if ($block instanceof Mage_Adminhtml_Block_Customer_Edit_Tabs) { // problem when 3rd-party overwrite the block
        if ($block->getId() == 'customer_info_tabs') {
            if (Mage::registry('current_customer')->getId() && Mage::getSingleton('admin/session')->isAllowed('customer/manage/logger')) {  
                $block->addTabAfter('changelog_grid', [
                    'label'     => Mage::helper('logger')->__('Changelog'),
                    'class'     => 'ajax',
                    'active'    => false,
                    'url'       => $block->getUrl(
                        '*/changelog_grid', ['_current' => true, 'prefix' => 'customer']
                    ),
                ], 'tags');
            }
        }
    }

[colinmollenhour]

I'd suggest either a rename for consistency:

Suggested change

	Mage::dispatchEvent('adminhtml_widget_tabs_add_after', array('block' => $this));
	Mage::dispatchEvent('adminhtml_widget_tabs_html_before', array('block' => $this));

Or, adding a new method to dispatch the event after the layout is prepared which is always where the default tabs are added:

    protected function _prepareLayout()
    {
        parent::_prepareLayout();
        Mage::dispatchEvent('adminhtml_widget_tabs_prepare_layout_after', array('block' => $this));
        return $this;
    }

[kiatng]

@colinmollenhour Thanks for your review! I tested adminhtml_widget_tabs_prepare_layout_after but the result is the same as the event core_block_abstract_prepare_layout_after; although the tab was rendered, it would set the added tab as active, irregardless of the param active value.

I corrected the event name to adminhtml_widget_tabs_html_before. My mistake seems obvious now but I was blindsided by the intended function I want.

[colinmollenhour]

Looks good! Thanks!