Unifying media controls and supporting playlists in the editor:

* Support a `caption` attribute for audio and video shortcodes
* In `wp.media.audio|video`, rename `update` to `shortcode` to allow these models to share the same mixins as `wp.media.collection` subclasses
* When sending an audio or video shortcode to the editor, create a default caption if the user hasn't entered one. This currently only displays in the editor, not on the front end. Captions aren't tied to a specific attachment here because external sources are supported.
* In the `wp.mce.media` mixin, in the `edit` method, read `attr` instead of `data` when attempting to parse the encoded shortcode. `data` does not automatically update when the attribute changes. This was a blessing to debug.
* Add `wp.mce.media.PlaylistView` to support playlist views in TinyMCE
* Expose `WPPlaylistView` to global scope and suppress auto-parsing of playlist nodes when in the admin. Allow `WPPlaylistView` to be passed `metadata` on creation instead of requiring a JSON blob to be parsed.
* Remove all of the playlist logic from the `wpgallery` TinyMCE plugin.
* In `wp_prepare_attachment_for_js()` return more data for audio/video so that playlists can have parity in the admin/front end.

See #27320.



git-svn-id: https://develop.svn.wordpress.org/trunk@27640 602fd350-edb4-49c9-b593-d223f7449a82