Support QGraphicsItems fixed to the view

Please add support for attaching items to the viewport, so that they remain unchanged and stay fixed / aligned with the viewport when the viewport is scrolled or transformed.

For example, games may display minimaps or status bars in the corner / bottom of the screen while the "scene" is scrolled.

One approach might be to add support for several scenes to a single view (e.g., "layers"), for which the viewport transform only optionally applies to each of the layers. If any of the scenes don't follow the viewport transform, the view would have to be forcefully switched to either NoViewportUpdate or FullViewportUpdate mode. For rendering and input handling, it might be enough to loop over all scenes in paintEvent and the mouse event handles...