Refinements for asynchronous rendering in `wp.mce.media.PlaylistView`:

* Add `visibility: hidden` as an inline style to `<audio>` tags, there is a race with the stylesheet which can get enqueued in the body and loaded in the footer.
* When creating new instances of `MediaElementPlayer`, always push them onto a stack. Lone views can be responsible for multiple instances of the same shortcode on render.
* Rename `wp.media.mixin.unsetPlayer()` to `wp.media.mixin.unsetPlayers()` to reflect the above.
* Call `wp.media.mixin.unsetPlayers()` on the view's `unbind()` method, instead of inline in the `render()` method
* Make sure `WPPlaylistView` is instantiated for each editor instance
* Ensure that the `No Items Found` view state is not rendered when attachments actually do exist.

Props gcorne, wonderboymusic.
See #27899.




git-svn-id: https://develop.svn.wordpress.org/trunk@28182 602fd350-edb4-49c9-b593-d223f7449a82