MetaWaylandKeyboard: use the new clutter-evdev support for xkbcommon

We need to track the full xkb_state to have the necessary information
to send to the clients, otherwise they may get confused and lock
or invert the modifiers. In the evdev backend, we just retrieve the
same state object that clutter is using, while in the other backends
we fake the state using what clutter is providing (which is a subset
of what X11 provides, which would be necessary to have full state)

https://bugzilla.gnome.org/show_bug.cgi?id=705862