Group header include

Header parts:

GAME_ERROR LoadGame(const std::string& url) override;
GAME_ERROR LoadGameSpecial(SPECIAL_GAME_TYPE type, const std::vector<std::string>& urls) override;
GAME_ERROR LoadStandalone() override;
GAME_ERROR UnloadGame() override;
GAME_ERROR GetGameTiming(game_system_timing& timing_info) override;
GAME_REGION GetRegion() override;
bool RequiresGameLoop() override;
GAME_ERROR RunFrame() override;
GAME_ERROR Reset() override;