Add core support for Playlists and Video Playlists.

* Playlists operate like galleries in the admin. 
* Provide default UI and JS support in themes using MediaElement and Backbone. 
* The shortcodes are clickable, editable, and configurable using the media modal. 
* Playlists support images for each item, whether or not the current theme supports images for `attachment:audio` and `attachment:video`
* Playlists respond to `$content_width` and resize videos accordingly.
* All playlist data is included inline, using a script tag with `type="application/json"`, allowing anyone to unenqueue the WP playlist JS and roll their own.
* Playlist styles are minimal and work out of the box in the last 5 default themes. They inherit and adapt to the current theme's font styles, and their rules are easily overrideable.

See #26631.



git-svn-id: https://develop.svn.wordpress.org/trunk@27239 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/css/media-views.css

@@ -595,10 +595,6 @@
 	box-shadow: 0 4px 4px -4px rgba( 0, 0, 0, 0.1 );
 }
 
-.media-frame .media-toolbar .add-to-gallery {
-	display: none;
-}
-
 .media-frame-title h1 {
 	padding: 0 16px;
 	font-size: 22px;
@@ -1427,7 +1423,7 @@
 	margin: 1.4em 0 0.4em;
 }
 
-.gallery-settings {
+.collection-settings {
 	overflow: hidden;
 }
 
@@ -1886,7 +1882,7 @@
 		border-top: none;
 	}
 
-	.gallery-settings h3 {
+	.collection-settings h3 {
 		margin-top: 45px;
 	}