New method of managing media selections. There is now a "master" selection in...

New method of managing media selections. There is now a "master" selection in the background. props koopersmith. fixes #22725

* This especially helps when you switch between insert/gallery and have non-images selected (as galleries don't support non-images).
* The views now act as filters on the "master" selection, instead of having their own selections that get passed around.

git-svn-id: https://core.svn.wordpress.org/trunk@23089 1a063a9b-81f0-0310-95a4-ce76da25c4cd