QGraphicsProxyWidget: forward touch events to QWidget

When working with QGraphicsView/QGraphicsScene, touch events are sent to
QGraphicsView's viewport() event handler, which then dispatches it to the
corresponding QGraphicsItem, if any. In the case of QGraphicsProxyWidget, we
need to forward these touch events to the encapsulated QWidget, otherwise it
will never receive them (i.e. the event chain for touch events terminates at
QGraphicsProxyWidget).

This also enables QWidgets associated with QGraphicsProxyWidget to grab
gestures.

Task-id: QTBUG-45737
Change-Id: Ia441d3576afb6c97376be6f2ff073901e6e928a5
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>