Merge FileFormats dll into Game and reorganise namespaces.

This unifies Game and FileFormats, and starts reorganizing things. A future cleanup may want to push the sprite and container definitions into the mod dlls, which would allow for modders to define their own loaders without touching the engine.

The justification for this is that a) this split hasn't been relevant for quite some time now (all our tools already linked against Game), and b) Having the Cell and World types in OpenRA.Game would dramatically simplify the lua rewrite.  FieldLoader depends on these types, so it had to come too -> FileSystem needed FieldLoader -> Format parsing code needed FileSystem.  The remaining bits aren't important enough to keep their own dll.