Remove graphic set from Level Header Editor / Add warning for incompatible graphic sets / ...

[LiraOnGithub]

You cannot easily change the graphic set of a level, only if the graphic sets are compatible. If you want level 1-1 to be a hilly level, the advice has been to use another level, like 1-2, and swap it with level 1-1.

That does make the graphic set setting in the Level Header Editor redundant and also confusing for newcomers.