Pass events to QGestureManager from the main (GUI) thread only.

Task-number: QTBUG-20946

Change-Id: I964452c1e97bd3d3a521e33abeb2f7d8cdbc89a9
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
(cherry picked from qtbase/9b28ab3f38e21f2e1498dea5dd761d22d72fb1ac)