TinyMCE: Differentiate active and hover states for toolbar buttons like the kitchen sink.

This restores 3.8 behavior.

see #27279.

Built from https://develop.svn.wordpress.org/trunk@27778


git-svn-id: http://core.svn.wordpress.org/trunk@27615 1a063a9b-81f0-0310-95a4-ce76da25c4cd