Logically move SystemBody generation to StarSystemGenerators.
Move SystemBody-related PopulateStarSystemGenerator methods to the correct file.
Move SystemBody-related StarSystemRandomGenerator methods to the correct file.
Move SystemBody-related StarSystemLegacyGeneratorBase methods to the correct file.
Remove some unused constants.
Move setting of SysPolit to PopulateStarSystemGenerator.
Determine commodity legality in PopulateStarSystemGenerator.
Merge pull request #3167 from lwho/galgen_part2

Galaxy generation, part 2