Change prototype of sdlmenu_open to remove dependency towards SDL
Handle "out of memory" error in sdlmenu_open()
Don't re-initialize SDL and SDL_ttf if already initialized
Don't pass parameters to the "flip" callback, as none is needed
Don't pass parameters to the "repaint" callback, as none is needed
Change prototype of sdlmenu_set_area to remove dependency towards SDL
Modify sdlmenu_set_text_color to remove dependency towards SDL
Remove last bits of SDL in the API