|SOURCE|
[Common::GameplayState]
* This is responsible for the main game part of the program
* AI, networking, and other tasks will be carried out in this class
[Common::GameStateManager]
* Execute checks if the stack is empty, if it is, then the game state manager
  will exit, otherwise it will continue as normal
[Common::IntroductionGameState]
* Exit calls GameStateManager's Pop member function to remove the current state
  from the state stack
[Common::MainMenuState]
* Exit calls GameStateManager's Pop member function to remove the current state
  from the state stack
* If the enter key is struck, the current state is replaced with the gameplay
  state