Avoid creating new devices from within the input thread which was the case for
tablet tools. It requires a lot more care about locking and has a potential to
mess up things.

Instead, schedule a WorkProc and buffer all events until we have the device
created. Once that's done, replay the event sequence so far. If the device
comes into proximity and out again before we manage to create the new device
we just ditch the whole sequence and wait for the next proximity in.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>