Unclean Level Shutdown

[alochmann]

When a level is deleted from memory (e.g. when the level ends or the app is shut down), level.reset() is called. This first sets the pointer "level" is pointing to to null, then deletes the world. However, we sometimes have to call DisposeObject during world deletion, which in turn refers to "level"; which has just been set to null. To avoid segfaults, we provide level with a new world instead. This method might have unexpected side effects: E.g. when unnaming an object during its deletion, it will search its name in the new world's catalogue instead of the old world's.

As of 2022, there are no known symptoms. Cleaning the code involves reworking all destructors. So we defer the repairs.