* Abstracts `wp.media.view.AttachmentDisplaySettings` into `wp.media.view.Settings` for managing lists of settings with button groups and select boxes. Settings can optionally be tied to a user setting (i.e. using `getUserSetting`).
* Adds `wp.media.view.Settings.AttachmentDisplay`.
* Adds `wp.media.view.Settings.Gallery`.
see #21390, #21815.
git-svn-id: http://core.svn.wordpress.org/trunk@22340 1a063a9b-81f0-0310-95a4-ce76da25c4cd