Merge sprite.cc and screen.cc

[zabidenhtf]

It can be helpful because now screen.cc it just interface for sprite.cc, and im think merge it is good idea

[petitlapin]

I started a bit in #80.
I'm not sure we can fully merge both. Conceptually, the sprites should be displayed on the screen (so, Sprite should not be a global variable, it should be an internal attribute of Screen class and the other classes should not have access to it, only the enum to ask what to display and where).

Looking more closely at the code, we already have free static method DrawText() in sprite.cc which should probably not exist and the text drawing should be done in screen.cc (it already exists).

I don't plan to do more now than the PR above, feel free to jump in if you want to refactor this case and remove the Sprite global variable :).