Compress touch events in QQuickWindow.

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>