Fix duplicate creation of window (makes fullscreen mode work on OSX)

Fix duplicate creation of window (makes fullscreen mode work on OSX)

Thanks to gdb, after hours of fidding, I finally found out why
this stupid fullscreen mode would not work: The code was instantiating
two instances of Window, both calling SDL_SetVideoMode and then the
second one would invalidate the first one, which is in turn used by
the drawing code to update the screen.

This has been fixed by creating the Window only once and passing the
Window object from the Game object to App, which can then pass it on
to all the other widgets and layers that need to draw to the screen.