status/keyboard: Synchronize input source switching with key events

Currently we simply set the gsettings key when activating an input
source. This obviously introduces a time window, between the event that
activates the switch and when the switch is complete, under which key
events are being delivered to applications and interpreted according
to the previous input source.

The patches in bug 696996 introduce a DBus API in g-s-d that allows us
to know when an input source if effectively active. Using that and
freezing keyboard events in the X server until we hear back from g-s-d
we can ensure that events won't be misinterpreted after an input
source switch.

https://bugzilla.gnome.org/show_bug.cgi?id=697007