Stricter special treatment of world names “TUTORIAL” and “DEFENSE”

Weird stuff happens if one creates the world “TUTORIAL” manually and then goes back and forth between tutorial and normal play. So this PR simply reserves the world names “TUTORIAL” and “DEFENSE”. “DEFENSE” is reserved for future use, in case the defense mode allows saving again in the future (I hope so!).

Additionally, in the main menu, the world names “TUTORIAL” and “DEFENSE” are in light cyan to emphasize that these savegames were not created manually.