Forward QGraphicsView::mouseDoubleClickEvent

Do the same as in mousePressEvent.
Otherwise it is not possible to handle the event
in one of the graphics views parents.

Task-number: QTBUG-8061
Change-Id: I67c7635361a9ed595c513c28ea016e6253fa2101
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>