Audio/Video shortcodes in the media modal:

* Make a generic model, `wp.media.model.PostMedia`, which replaces `wp.media.model.PostAudio` and `wp.media.model.PostVideo`
* Make a generic library, `wp.media.controller.MediaLibrary`, which replaces `wp.media.controller.ReplaceVideo` and `wp.media.controller.ReplaceAudio`
* `wp.media.controller.MediaLibrary` is used to create new states that want to load a library filtered by type, making it incredibly simple to add states to frames. See `wp.media.view.MediaFrame.VideoDetails` and `wp.media.view.MediaFrame.AudioDetails`.

UX changes:

* Add the ability to manage HTML5 fallbacks - add additional `<source>`s or remove specific `<source>`s
* In the Video Details state, add the ability to select a poster image

See #27016.



git-svn-id: https://develop.svn.wordpress.org/trunk@27478 602fd350-edb4-49c9-b593-d223f7449a82