Add support for opening submenus when parent has focus

As mentioned in an e-mail a while back, I modified the menu to be keyboard accessible.

It works for 1-3 levels (not tried deeper). I'm not sure if it's the best method, but does seem to work pretty well. It works by finding all its parents and giving them the `openSubmenuClass` class, which is used by to toggle visibility of the submenus when viewed on a mobile device.