Fix global fullscreen shortcut in SDL2

Keycode for enter in SDL2 is "Return" not "enter"
http://wiki.libsdl.org/SDL_Keycode

I set the SAVEGAME_VERSION to 83.  This will have to be bumped if we have more SAVEGAME_VERSIONs in the master branch.  If that happens I think we should just bump the SDL2 branch to 100.