Added a splash screen game state

|SOURCE|
[Common/SplashScreenState]
* The splash screen state is used as the introduction to the game, before the
  main menu, currently it sets the clear colour to that of the Dreamcast boot
  sequence background colour and transitions into the gameplay state after one
  second
[Common/Game]
* Created and added the SplashScreenState to the GameStateManager
* SplashScreenState is the first state used
[Common/GameplayGameState]
* Exit calls GameStateManager's Quit member function to exit the game
  completely
* The event router is set to be the global event router