Restore the ability to pass classes to admin menu submenu items, important for the Customize link. Accidentally left out in [24048]. fixes #28583.

Built from https://develop.svn.wordpress.org/trunk@28778


git-svn-id: http://core.svn.wordpress.org/trunk@28591 1a063a9b-81f0-0310-95a4-ce76da25c4cd