Fix crazy use of RefCountedPtr in StarSystemCache.
Basic GalaxyGenerator infrastructure
Move Sector generation code to SectorGenerators.
Basic StarSystem generation infrastructure in GalaxyGenerator.
Move StarSystem generation code to StarSystemGenerators.
Avoid StarSystem's excessive list of Generator friends with a variant of the Attorney-Client pattern.
Use the Attorney-Client pattern for GalaxyGenerator access as well.
Move Populate to a separate StarSystemGenerator.
Move the knowledge how to assemble a galaxy generator from Pi to a static method of GalaxyGenerator.