Big update for Checkers.

- There are many changes, debugs and improvements for checkers, please refer to specific commits and try it!
- There are other changes to other games too.
- Structure of the libraries (Generic, TTT and Checkers) should be good to further enhance comments based on current layout.
- There are one or two known bugs in checkers which will be fixed soon, should not prevent anything for now. (e.g. Agent rarely reads a response as "null".)