Forward declared SDL_Surface is a struct not a class.
Basic GalaxyGenerator infrastructure
Move Sector generation code to SectorGeneratorStages.
Move StarSystem generation code to StarSystemGeneratorStages.
Avoid StarSystem's excessive list of Generator friends with a variant of the Attorney-Client pattern.
Move Populate to a separate StarSystemGenerator.
Rudimentary savegame support for galaxy generator (bumps saveVersion).

Currently this only stores galaxy generator name and version on save
and verifies that they match (same name, version less or equal) on
load.

When this is completely implemented, the selected galaxy generator
must be created and factions and such regenerated if a change of
generator occured.

But currently we only have one generator.