Fix the tabbing JS to work as described in the W3 ARIA draft: Tab to select,...

Fix the tabbing JS to work as described in the W3 ARIA draft: Tab to select, Enter to show submenu, Esc to close submenu, return to top menu and focus it. See #19394

git-svn-id: https://core.svn.wordpress.org/trunk@19483 1a063a9b-81f0-0310-95a4-ce76da25c4cd