xcb: touchpad support

Relative touch devices tend to provide a moving point for one finger,
then when more points are pressed, the root_x/root_y stay stationary
while the Rel X/Y valuators provide info about the movements around
that point.  Synthesize moving touch points from those so that
gestures become possible.  Behavior is now similar to that on OS X.

Change-Id: I1c65fb04849d19f11c3bf166044f296d95c33da1
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>