Partially fixes#2788. This change makes it so the lock screen is
redrawn whenever the caps lock modifier state changes, rather
on relying on the keypress event. This didn't work because
caps lock is disabled when the key is released, not pressed,
so the caps lock indicator does not go away until the next
keypress event.
The whole state->xcb.modifiers thing didn't work at all (always 0)
The xkb doc says "[xkb_state_serialize_mods] should not be used in
regular clients; please use the xkb_state_mod_*_is_active API instead"
so here it is
