Widgets: Introduce media widgets for images, audio, and video with extensible base for additional media widgets in the future.

The last time a new widget was introduced, Vuvuzelas were a thing, Angry Birds started taking over phones, and WordPress stopped shipping with Kubrick. Seven years and 17 releases without new widgets have been enough, time to spice up your sidebar!

Props westonruter, melchoyce, obenland, timmydcrawford, adamsilverstein, gonom9, wonderboymusic, Fab1en, DrewAPicture, sirbrillig, joen, matias, samikeijonen, afercia, celloexpressions, designsimply, michelleweber, ranh, kjellr, karmatosed.
Fixes #32417, #39993, #39994, #39995.


git-svn-id: https://develop.svn.wordpress.org/trunk@40640 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/customize-selective-refresh.js

@@ -469,6 +469,15 @@ wp.customize.selectiveRefresh = ( function( $, api ) {
 			// Prevent placement container from being being re-triggered as being rendered among nested partials.
 			placement.container.data( 'customize-partial-content-rendered', true );
 
+			/*
+			 * Note that the 'wp_audio_shortcode_library' and 'wp_video_shortcode_library' filters
+			 * will determine whether or not wp.mediaelement is loaded and whether it will
+			 * initialize audio and video respectively. See also https://core.trac.wordpress.org/ticket/40144
+			 */
+			if ( wp.mediaelement ) {
+				wp.mediaelement.initialize();
+			}
+
 			/**
 			 * Announce when a partial's placement has been rendered so that dynamic elements can be re-built.
 			 */