Galaxy [4/4]: Move Sector and StarSystem caches to Galaxy

** Depends on parts #2890, #2891, #2892; to be rebased and merged afterwards **

And the last part of the series, which move the ```Sector``` and ```StarSystem``` caches to ```Galaxy```.

I'm a bit unsure if we should merge the parts separately (as the PRs suggest) or just make a single merge of this last one (which contains all the other ones).