Instead of sending multiple touch updates per frame, we
store the last one and flush the pending events just before
we enter into the scene graph sync phase.

[ChangeLog][QtQuick] QQuickWindow will compresses touch events
and delivers at most one touch event per frame.

Done-with: Robin Burchell <robin.burchell@jollamobile.com>
Change-Id: Ia0169bc4a3f0da67709b91ca65c326934b55d372
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>