`wp.media.collection` should be its own civilized instantiable class, not a wrapper/factory. The class shall contain no reference to specific instances, and shall not try to grab static properties of itself. `self`, meet `this`.

See #26631.



git-svn-id: https://develop.svn.wordpress.org/trunk@27313 602fd350-edb4-49c9-b593-d223f7449a82