ScrollingWorld

This extended world may be used to:
- Navigate an 'InterActor' in a large world.
- Move a background with the current View.
- Allow objects to act/live outside the View.
The ScrollingWorld replaces may first version
of ScrollWorld.
Use the following move-command keys:
- A,W,S,D: move the view respectively in a left, up, down or right position.
- Arrow keys: move the InterActor in the corresponding arrow key direction.
- C: center the view around the InterActor's position
- K: set movement to (slow) keypress-mode.
- M: set movement to (fast) keydown-mode.