Move SystemBody creation to StarSystem generators

This is a followup PR to the galaxy generator merge.

It moves the ```SystemBody```-related generation code to the appropriate ```StarSystem``` generators.

I considered creating a separate type of generator for ```SystemBody```, but this wasn't feasible as ```StarSystem``` and ```SystemBody``` generation is tightly integrated.

As before, this is a pure refactoring and shouldn't change observable behaviour.

The first commit only moves the methods logically to their new classes, but keeps the code in their original file. That way, the diff properly reflects the applied changes. The next three commits are copy and paste from one file to another one.