Introduce removed event for wp.customize.Values collection

[westonruter]

Notifications were added to Customizer controls in #34893. Notifications are re-rendered when the control.notifications collection (a wp.customize.Values) has a wp.customize.Notification added or removed with the code as follows (via wp.customize.Control#initialize):

// After the control is embedded on the page, invoke the "ready" method.
control.deferred.embedded.done( function () {
        /*
         * Note that this debounced/deferred rendering is needed for two reasons:
         * 1) The 'remove' event is triggered just _before_ the notification is actually removed.
         * 2) Improve performance when adding/removing multiple notifications at a time.
         */
        var debouncedRenderNotifications = _.debounce( function renderNotifications() {
                control.renderNotifications();
        } );
        control.notifications.bind( 'add', function( notification ) {
                wp.a11y.speak( notification.message, 'assertive' );
                debouncedRenderNotifications();
        } );
        control.notifications.bind( 'remove', debouncedRenderNotifications );
        control.renderNotifications();

        control.ready();
});

Notice the multi-line comment. We can't re-render the notifications at a remove event because this event gets triggered just before it is removed, somewhat unexpectedly. We should therefore introduce a removed event that gets triggered after the removal so that workarounds using debounce or defer aren't required.

Submitted patch should include unit test.

Can still make 4.7 if there's a patch, but we need a patch for now.

@westonruter - What do you think of such implementation:

​https://github.com/xwp/wordpress-develop/pull/193

@delawski nice. I added a code review comment.

@westonruter Thank you for the review. I've added the condition as you suggested and also wrote additional unit test to cover case you've mentioned in the comment.

Excellent. This can go into 4.8 as soon as trunk it open for enhancements.

I'm going to add this as part of #35210.

Merged into ​https://github.com/xwp/wordpress-develop/pull/180 as ​https://github.com/xwp/wordpress-develop/pull/180/commits/8bbe1f12e2a27bd143b3df3fada3e377ec92f6cf

In 41374:

Customize: Add global notifications area.

• 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.