TinyMCE: Restore old wordpress_adv_hidden editor parameter to enable force-showing the kitchen sink.

props azaozz.
see #27963, for trunk.

Built from https://develop.svn.wordpress.org/trunk@28181


git-svn-id: http://core.svn.wordpress.org/trunk@28012 1a063a9b-81f0-0310-95a4-ce76da25c4cd