Avoid duplicate emulated mouse events with XInput2

When using a touch screen on a Linux machine, we receive both touch-events
and emulated mouse events from XInput, on top of that we synthesize mouse-
events ourselves for the touch events.

This patch grabs the touch device for touch events whenever it processes
a touch-begin thereby avoiding XInput from synthesizing mouse events.

Task-number: QTBUG-35157
Change-Id: I5849d5841be236d6719cd080af2e9e39eb9cdd84
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>