Bring Featured Images back into the main media dialog.

Most users don't realize that the Featured Image meta box exists; if they do, few use it.

Restores the old meta box UI, including the admin_post_thumbnail_html filter. If a plugin is using _wp_post_thumbnail_html() in conjunction with Thickbox elsewhere, it will also magically still work.

Specific underlying changes:
 * Converts the modal view to use the view manager, which means that a call to open() will automatically call render and attach if necessary.
 * Doesn't automatically set a state in wp.media, to allow code to customize the states to be added before activation.

props koopersmith.
fixes #21776.



git-svn-id: https://core.svn.wordpress.org/trunk@22979 1a063a9b-81f0-0310-95a4-ce76da25c4cd