Galaxy [1/4]: Convert the Galaxy namespace to a class

Seems I have a productive weekend ;)

This is the first PR in a series of four to come. It converts the ```Galaxy``` namespace to a class so we can properly encapsulate everything that reflects our generated galaxy (and thus changes when different galaxy generators are used):

1. The sector density map (was already part of the ```Galaxy``` namespace)
2. The factions
3. The custom systems
4. ```Sector``` and ```StarSystem``` caches