Rather than extending `wp.media.mixin`, only borrow `coerce` when necessary in `wp.media.audio|video|collection`. `wp.media.mixin` will make sense to be mixed in for classes that expect to interact with the player. More universal methods can be added and inherited by all those who extend their prototype with it.

Assuming someone had implemented players in the editor, pause all players when switching between editor tabs. A method, `pauseAllPlayers`, has been added to `wp.media.mixin`.

See #27389.


Built from https://develop.svn.wordpress.org/trunk@27537


git-svn-id: http://core.svn.wordpress.org/trunk@27380 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Move `removePlayer()` and `unsetPlayer()` to `wp.media.mixin` so that other classes can take advantage of them. 

* `removePlayer()` is an alternative to MediaElement's player removal method that does not re-add the audio|video element back to the DOM.
* `unsetPlayer()` will check for an existing `player` property on the passed instance and pause all players before calling `unsetPlayer()`

See #27389.


Built from https://develop.svn.wordpress.org/trunk@27538


git-svn-id: http://core.svn.wordpress.org/trunk@27381 1a063a9b-81f0-0310-95a4-ce76da25c4cd