Widgets: Add dirty state tracking for widgets on admin screen.

* Mark a widget as dirty when a field input triggers a `change` or `input` event; clear dirty state when widget is successfully saved.
* Disable Save button and re-label "Saved" when widget not dirty.
* Show AYS dialog when leaving widgets admin screen with unsaved changes.
* When widgets are dirty, expand all unsaved widgets at AYS check and focus on first one.
* Change "Close" link to "Done"; hide link when widget is dirty and reveal when saved.
* The "Done" link persistently appears in the Customizer even after making a change (when the widget is dirty) because changes are autosaved into the changeset.
* Prevent saving widget when form fails `checkValidity`.
* Fix frequency of triggering of `change` event on the rich Text widget's `textarea` limited now to when there are actual changes.
* Add a class of `widget-dirty` to widget containers when the widget has unsaved changes.

Props westonruter, timmydcrawford, melchoyce.
Fixes #41610, #23120.

Built from https://develop.svn.wordpress.org/trunk@41352


git-svn-id: http://core.svn.wordpress.org/trunk@41185 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/widgets.css

@@ -42,6 +42,10 @@
 	line-height: 16px;
 }
 
+.widget.widget-dirty .widget-control-close-wrapper {
+	display: none;
+}
+
 .in-widget-title,
 #widgets-right a.widget-control-edit,
 #available-widgets .widget-description {