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

props matveb.
see #27055. fixes #27695.

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git-svn-id: http://core.svn.wordpress.org/trunk@27855 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Improve hook docs for the `wp_enqueue_editor` hook, added in 3.9.

See #16284, #27700.

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git-svn-id: http://core.svn.wordpress.org/trunk@27856 1a063a9b-81f0-0310-95a4-ce76da25c4cd