Widgets: Trigger jQuery events for widget updates.
* `widget-added` when a widget is added to a sidebar
* `widget-updated` when a widget is updated
A jQuery object of the widget is passed along to the event handler. Same events are used in the Widget Customizer, see [27909].

fixes #19675.

git-svn-id: https://develop.svn.wordpress.org/trunk@27969 602fd350-edb4-49c9-b593-d223f7449a82