keybindings: Add MetaKeyBinding for overlay-key

As the overlay key works differently from normal keybindings, it
requires special treatment. However, by adding a rudimentary
MetaKeyBinding for it, we will be able to confine the special
handling to mutter and treat it like any other keybinding in
the shell.

https://bugzilla.gnome.org/show_bug.cgi?id=688202