Backward compatibility for 0.6.3 -> object types

[tobbi]

In 0.6.3, the type of objects and badguys was determined by the sprite path. In latest master, this has changed significantly.

In order to keep backward compatibility, I'm therefore proposing checking for the sprite name in old level files and setting an object type based on that earlier sprite path unless an object type is already set.

I'm going to think about how to best accomplish this.