MetaWaylandSeat: don't use use events to count pressed buttons

Use the modifier mask instead, as events can get lost if there
is a clutter grab or if some other actor is capturing events.

https://bugzilla.gnome.org/show_bug.cgi?id=706124
wayland: reimplement keyboard state handling properly

We can't rely on clutter's xkb_state, because that's updated
when events are pulled from the kernel, not when we see them.
Instead, use the new clutter API to get the full modifier state
from the event (which, as a side effect, also works when clutter
is using the X11 backend for running nested).

https://bugzilla.gnome.org/show_bug.cgi?id=706963