[Core] Add CMake source groups to target 'game' [c2058]

This is the exact same grouping as it is currently in the VC 2012 files.
These groupings will have to be refactored at some point as they are not
very logical.

(based on commit [12543] - 0088ce3)

Signed-off-by: Daniel S. Reichenbach <daniel@kogitoapp.com>