dix: Rework Enter/Leave semaphore system.

Instead of a simple counter, use bits to keep track of which device is where
etc. When device enters a window (or sets focus), the bit matching the device
is set, when it leaves again, it is unset. If there are 0 bits set, then
Leave/Enter/Focus events may be sent to the client.

Same theory as before, but this should get around the insanity with
Grab/Ungrab special cases. Those cases are basically untested though.