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/class-wp-customize-section.php

@@ -361,6 +361,8 @@ class WP_Customize_Section {
 								{{{ data.description }}}
 							</div>
 						<# } #>
+
+						<div class="customize-control-notifications-container"></div>
 					</div>
 
 					<# if ( data.description && ! data.description_hidden ) { #>