Added some and implemented some game state related things to the Game class.

 - Included "GameState.h" in Game.h
 - void Game::setGameState(std::shared_ptr<GameState>) noexcept; (Sets the game state as well as signals it was changed, implemented and documented)
 - std::shared_ptr<GameState> Game::gameState_; (The current game state, documented)
 - bool Game::gameStateChanged_ = false; (Whether or not the most recent game state change has been handled by the game loop, set by Game::setGameState(), documented)
Fixed up some documentation, specifically backtick quoted SDL function references as well as added parentheses to them.