Add a "Beta Media" button to the post editor. Currently, it is only capable of inserting images. Other attachment types and galleries need not apply... yet.
* Added `wp.media.string.image( attachment, props )` for generating an image as a string from an attachment and relevant attachment display properties.
* Properly localized the gallery workflow.
* Added `Workflow.update()`, which closes the modal, triggers an `update` event, and resets the selection.
* Added `wp.mce.media` to manage the various media workflows for editors.
see #21813, #21814, #21390.
git-svn-id: http://core.svn.wordpress.org/trunk@22036 1a063a9b-81f0-0310-95a4-ce76da25c4cd