Release the mouse grab from PinchArea when it's not needed anymore

If we keep the mouse grabbed no other items will receive mouse events
until the next click.

Task-number: QTBUG-37485
Change-Id: Id3936fbe09b262ddbefc68334ef8fe7e161fbcd4
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>