Don't forcibly accept touch events if mouse event synthesis is disabled

Instead of always accepting touch events, do that only if we are
generating mouse events for non accepted ones. This enables
QQuickWindow subclasses to detect if a touch event was indeed
handled by QQuickWindow.

Change-Id: I73512fb3597131c2296d6cdc7b3d5c8454b159b6
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>