The first loading screen based on the circle is now commited
add the StartScreen game state

- code `startGame` by creating `Gameloop`, `GameStateStack`
  and other useful stuff.
- add the StartScreen class
  it quit on `KEY_SPACE` event.
  Not yet a real draw method.
add 2 empty GameState classes `MapLoadingScreen` and `MapScreen`
moving all `*Screen` classes to `*State`, for better semantic.
add `MapLoadingState` logic and create stub classes for dependencies.

- complete the `MapLoadingState` class.
  Load the map file, give them to the TMX parser,
  and load the ressources required by the map.
- add a `MapLoadingScreen` class, which display the view corresponding to this state.
  Actually, no display, but some logs in the console.
- add an empty TMXParser class.