Customize: Add notifications API to sections and panels.

* Adds a `notifications` property to instances of `wp.customize.Panel` and `wp.customize.Section`.
* Adds a `setupNotifications()` method to `Panel`, `Section`, and `Control`.
* Adds a `getNotificationsContainerElement()` method to the `Panel` and `Section` classes, like `Control` has.
* Replace hard-coded notification in header media section with a notification.
* Limit rendering notifications to panels and sections that are expanded, and to controls that have an expanded section.

See #34893, #35210, #38778.
Fixes #38794.


git-svn-id: https://develop.svn.wordpress.org/trunk@41390 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/script-loader.php

@@ -563,6 +563,7 @@ function wp_default_scripts( &$scripts ) {
 		'expandSidebar'      => _x( 'Show Controls', 'label for hide controls button without length constraints' ),
 		'untitledBlogName'   => __( '(Untitled)' ),
 		'serverSaveError'    => __( 'Failed connecting to the server. Please try saving again.' ),
+		'videoHeaderNotice'   => __( 'This theme doesn\'t support video headers on this page. Navigate to the front page or another page that supports video headers.' ),
 		// Used for overriding the file types allowed in plupload.
 		'allowedFiles'       => __( 'Allowed Files' ),
 		'customCssError'     => wp_array_slice_assoc(