Galaxy generator

Just a very short introduction: This PR is supposed to separate galaxy generation (galaxy, sectors and star systems) from their state making it possible to experiment with new ways to generate the galaxy (see also http://pioneerspacesim.net/forum/viewtopic.php?f=4&t=133 ).

Current state:
- [x] Basic infrastructure: Sector and StarSystem generators consisting of pluggable generator stages (though the "pluggable" has to be taken with a big grain of salt. The stages are not as decoupled as they could be).
- [x] Moved current Sector generation code to generator
- [x] Moved StarSystem-level system code to generator
- [ ] **TBD: Move SystemBody generation to generator**
- [x] Basic savegame support (saving used generator to savegame and verifying it is compatible with code at load).
- [x] Switch galaxy generators on game load/start if needed

The scope of this PR is pure refactoring, i.e. giving the galaxy generator code a new structure without changing it functionally.

The branch might still be **history-edited** (rebased, commits squashed, reordered).

I had to change this to `protected:` locally to get it to compile. `StarSystem::GeneratorAPI` inheriting from `StarSystem` can't see `NewBody` otherwise. Alternatively you could move `NewBody` into a protected section.