Synchronize QInputDeviceManager touch device count with QTouchDevice

This ensures that the values and signals reported by QInputDeviceManager
for touch devices always have corresponding entries in the list returned
by QTouchDevice::devices().
It also adds proper QTouchDevice unregistration when the underlying
input device gets removed by the evdevtouch QPA plugin.

Change-Id: I7bdf2f7435c775d15bddce8ba1e731afdc1d948a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>