* Displays an error notification in the global area when a save attempt is rejected due to invalid settings. An error notification is also displayed when saving fails due to a network error or server error.
* Introduces `wp.customize.Notifications` subclass of `wp.customize.Values` to contain instances of `wp.customize.Notification` and manage their rendering into a container.
* Exposes the global notification area as `wp.customize.notifications` collection instance.
* Updates the `notifications` object on `Control` to use `Notifications` rather than `Values` and to re-use the rendering logic from the former. The old `Control#renderNotifications` method is deprecated.
* Allows notifications to be dismissed by instantiating them with a `dismissible` property.
* Allows `wp.customize.Notification` to be extended with custom templates and `render` functions.
* Triggers a `removed` event on `wp.customize.Values` instances _after_ a value has been removed from the collection.
Props delawski, westonruter, karmatosed, celloexpressions, Fab1en, melchoyce, Kelderic, afercia, adamsilverstein.
See #34893, #39896.
Fixes#35210, #31582, #37727, #37269.
git-svn-id: https://develop.svn.wordpress.org/trunk@41374 602fd350-edb4-49c9-b593-d223f7449a82
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