Split FrEee.Core into a hierarchy of projects

[ekolis]

What needs to be cleaned up?
FrEee.Core (or just FrEee) is a huge project with lots of namespaces and classes in it. It would help me understand how everything links together if I could split it into multiple projects, such as Map, Civilization, and Modding, each of which can reference projects lower in the hierarchy but not higher, e.g. Map can reference Civilization and Modding, but Modding can't reference Civilization or Map - and immediately that presents a problem as mod templates will need to be able to reference the object types that they create!

How will this benefit us?
Make the code structure easier to understand in order to enable further refactorings and rewrites (e.g. better understand the hierarchy of space object types to eliminate the referrables list).

What potential drawbacks are there to making this change?
Time consuming; might not be feasible if there are too many crosslinks

[ekolis]

This is getting messy, everything refers to everything else and I can't figure out where to split it up...

[ekolis]

See also #312 for factoring out some data models