When we get a ClutterModifierType from Clutter, it might contain
bits not in the enumeration. See bug 59771 for a similar problem
with GdkModifierType.
Add a wrapper Shell.get_event_state() around clutter_event_get_state()
to mask these bits out and only return approved bits.
https://bugzilla.gnome.org/show_bug.cgi?id=597735