Added a main menu state

|SOURCE|
[Common/GameStateManager]
* Initialise calls the base Initialise member function and checks if the
  renderer is not null
[Common/MainMenuState]
* The first state to be entered, this allows the user to select the demo scene
  to display as well as exit
* Upon Exit being called, the GameStateManager's Quit member function is called
[Common/Game]
* Removed the Update and Render calls in the main loop
* Using the MainMenuState for the GameStateManager