Add an abstraction of the gallery code in `media-editor.js` called `wp.media.collection`. This will be the basis for parsing `[gallery]`-like shortcodes in the media editor (thing playlists, collections of PDFs, etc).

There are currently no instances of this. Those will be forthcoming.

See #26631.



git-svn-id: https://develop.svn.wordpress.org/trunk@27212 602fd350-edb4-49c9-b593-d223f7449a82