consistency: remove borders around game canvases

Most games already have a game canvas without margins and frames. This
commit removes these borders everywhere. The main motivation is
consistency, but I also think that no borders have other advantages:

1. They look much better with borderless window decorations.
2. Frames induce an additional rendering cost which can be avoided.
3. Frames are usually used for visual grouping, but this is not
necessary for a game canvas with its intense themed appearance.
4. QGraphicsView has problems with establishing a 1:1 relation between
scene and view coordinates when it has a frame.
5. Also 4px horizontal and vertical space saved!!!! ;-)

If you are the maintainer of one of the affected games and object to
this change, please let me know.