Text editors: Prevent autoscroll timer triggering from synth mouse events

The autoscrolling interferes with scrolling pan gestures
and causes the scroll direction to be reversed when moving
outside the window.

Task-number: QTBUG-40461
Change-Id: I30ef848a346418929540c23730ab92f44e4565e2
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>