This fixes an issue in that when running under X11, when a modifier key
is pressed, the modifier state will "lag" behind what should be current.
This is due to how X11 sends modifiers in events, i.e. it sends the
state from right before the key press, and does not include the effects
of the key press itself.
This is corrected by checking the X event queue directly for a pending
XkbStateNotify event (we mask this on modifiers), and setting the
modifiers off of that if we find one. If not, we fall back to the GDK
call.
fc4fdbb643