Accessibility revamp for nav menus.

props lessbloat. fixes #14045

git-svn-id: http://core.svn.wordpress.org/trunk@23727 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Compress scripts/styles: 3.6-alpha-23728.

git-svn-id: http://core.svn.wordpress.org/trunk@23728 1a063a9b-81f0-0310-95a4-ce76da25c4cd