Partial wrapper for SDL 2.0 and SDL_image 2.0

This PR establishes the structure of the SDL 2.0 project which can be completed as needed with the development of games and frameworks.

Some more subsystems should be wrapped, and most should be completed. The event subsystem is implemented partially as I'm not sure of the form that it will take in the future frameworks. The audio and the ttf subsystems could be useful. Some other subsystems should probably not be wrapped: the platform compatibility layers offers mostly the same features as the Nit standard library, and the OpenGL functions are already covered by other Nit modules.
is_initialized?