Get rid of global galaxy object

Yet another "small" refactoring before I (re-)implement the first feature on the new ```Galaxy``` infrastructure.

This one, gets rid of ```Pi::galaxy```, mostly giving objects their own reference to the galaxy they belong to. The lifetime of the galaxy in now maintained by ```Game```. To avoid long game loading/starting times there is still a cached static ```Galaxy``` object hidden in ```GalaxyGenerator```. But it behaves as-if a new galaxy is generated every time you call ```Create```. I plan to completely get rid of this "permanent" galaxy rather sooner than later.

**No visible/behaviour changes intended.**