* Adds `wp.media.model.Selection.single()` to specify a single item used in a multi-item selection. * Fixes a bug where the `details` class would not be removed when "Clear Selection" was clicked. see #21390. git-svn-id: http://core.svn.wordpress.org/trunk@22335 1a063a9b-81f0-0310-95a4-ce76da25c4cd