CustomSystemsDatabase gives out only constant CustomSystems.
Const-correctness for Factions/FactionsDatabase.
restrict FreeType headers to a single translation unit (TextureFont.cpp)

This means we can remove some ugly #undefs from other files, and generally
avoid polluting the global namespace so much.

Fixes #1203
Merge remote-tracking branch 'lwho/const_correctness'
Merge remote-tracking branch 'jpab/issue-1203'