QGraphicsView - move rubberband-handling into a private function

This patch moves the rubberband-handling from QGraphicsView::mouseMoveEvent
to QGraphicsViewPrivate::updateRubberBand. This function is then
called from QGraphicsView::mouseMoveEvent.

I have removed some d-> and added some q-> but beside that
nothing is changed in the code.

Change-Id: Iab70c55635c43733e0e02bb70e2bb03b90bf62f0
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>