Easier way to add widgets in addons

[duncanmcclean]

I've done some source diving in the codebase around widgets, as the documentation on it is yet to be written.

Right now you need to do something like this to register a widget (although I haven't actually managed to get the last part working (the register extensions in app folder part)

$this->app->instance('statamic.widgets', collect());

$this->app['statamic.widgets']['recent_orders'] = RecentOrdersWidget::class;

$this->registerExtensionsInAppFolder('Widgets', 'statamic.widgets');

What I was thinking is that it would be easier to register the same way that tags and listeners are registered, simply like this.

protected $widgets = [
       RecentOrdersWidget::class,
];

I'm happy to have a look at implementing this in a pull request if you want 😃