Rescue Rover level 2 almost works

- Rover follows player
- Rover levels succeed when player reaches exit with Rover
- Player picks up items
- Keys unlock doors

Only thing missing is that the key should be removed from the level when
it is used.

This currently causes an error, as it happens in a loop that includes
the key.

Also changed a lot of the sprite fetching functions in the level class
to return sets.