This simply subclasses QGraphicsScene and is used as
a drop-in replacement. The plan is to step-by-step
move rendering functions there until the non-interactive
code can only use the scene and doesn't have to use
the QGraphicsView. This will hopefully remove quite
some conditional code.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
