Merge pull request #3193 from lwho/more_cache_unification

More cache unification
Update Changelog.txt
Factions code no longer uses global Galaxy object.
CustomSystems code no longer uses global Galaxy object.
Galaxy generator and caches no longer use global Galaxy object.
Fix Galaxy object searching itself via Pi::GetGalaxy().
Space no longer uses global Galaxy object.
Merge two constructors of Game to avoid another Pi:GetGalaxy reference.
Pass galaxy to StarSystem::Unserialize.
Give Game a m_galaxy member.
GalacticView, SectorView, SystemView, SystemInfoView and WorldView no longer use global Galaxy object.
Polit no longer accesses global Galaxy.
Make sure a GalaxyObjectCache is empty when it is destroyed.
Use explicitly passed galaxy for dumping.
StarSystem-related functions no longer use global Galaxy object.
Switch LuaSystemPath from Pi::GetGalaxy to Pi::game->GetGalaxy.
Store reference to Galaxy in Sector.
Replace remaining references of Pi::GetGalaxy by Pi::game->GetGalaxy.
Completely move Galaxy object to Game.