Now class GameGrid distinguishes between player's "head" and wall.
Now the game is executed on a GrameGrid object.
Now the game's background is drawn in a different color so it is distinguished from the game's grid.
Now class GameGrid depends on screen's dimensions.

- GameGrid's constructor throws if the given size is greater than the screen's dimensions.
- Players' initial positions are now initialized depending on GameGrid's dimensions.
- GameGrid's constructor requires a screen (SDL_Surface) as an argument, so all the drawing is performed on this screen
  (the screen argument has been removed from method GameGrid::draw().
- Game's grid is now drawn centered in the screen surface.