debug.hpp: Introduce TTCARDS_DEBUG_GAME_CONFIG preprocessor

* Disable the debugging unserialization output of GameConfig class.
GameConfig: Replace JsonCppValue with Value saving & loading

* (Finally!) Save functionality is once again restored.

* Introduce TTCARDS_DEBUG_GAME_CONFIG_LOAD &&
TTCARDS_DEBUG_GAME_CONFIG_SAVE preprocessors.

* Remove a bunch of old, obsolete code.