Reworked fullscreen policy on Mac OS X.

- SDL_WINDOW_FULLSCREEN works as always (change resolution, lock to window).
- SDL_WINDOW_FULLSCREEN_DESKTOP now puts the window in its own Space, and
  hides the menu bar, but you can slide between Spaces and Command-Tab between
  apps without the window minimizing, etc.
- SDL_WINDOW_RESIZABLE windows will get the new 10.7+ "toggle fullscreen"
  window decoration and menubar item. As far as the app is concerned, this is
  no different than resizing a window, but it gives the end-user more power.
- The hint for putting fullscreen windows into the Spaces system is gone,
  since Spaces can't enforce the requested resolution. It's a perfect match
  for FULLSCREEN_DESKTOP, though, so this is all automated now.