Theme Installer: Handle currently installed themes, add search route, let prev/next refresh collections.

props matveb.
see #27055. fixes #27695.


git-svn-id: https://develop.svn.wordpress.org/trunk@28025 602fd350-edb4-49c9-b593-d223f7449a82
Improve hook docs for the `wp_enqueue_editor` hook, added in 3.9.

See #16284, #27700.


git-svn-id: https://develop.svn.wordpress.org/trunk@28026 602fd350-edb4-49c9-b593-d223f7449a82