It however posts a bogus button up event once in a while. Whenever
we receive dx=dy=dz=buttons=0 we postpone adding it to the queue for
50msecs with a timeout. If in the meantime something else is posted
the event is ignored.
This avoids the problem Nik Sayer reported. He noticed that X windows
would drop and pick up a window once in a while.
Thanks, Nik, for supplying me with the keyboard to fix the problem!