Admin bar: change the tabbing to behave as if the admin bar menus were...

Admin bar: change the tabbing to behave as if the admin bar menus were click-to-open (Tab to select, Enter to open/close), fixes #19088

git-svn-id: https://core.svn.wordpress.org/trunk@19458 1a063a9b-81f0-0310-95a4-ce76da25c4cd