Move random biome system to a separate mapgen library.

Proposed library and directoryname agreed with FeXoR.
Make the rmgen/ biome agnostic by moving the biome constants in createForests to the randombiome globals.
Don't pass a non-randombiome constant in gulf of bothnia to the createForests function that expected a randombiome constant, refs rP14837, accepted by Grugnas.
Remove unused trivial unrecommended randomizeBiome function,
make Biomes autocompletable in the gamesetup chat, refs D852.