Move quit to menu to Game::quittomenu()

I don't want the quit code to only be in the state machine, but I'll
keep the gamestate around for compatibility reasons (there are custom
levels that directly use gamestate 80 to quit to the menu).

desktop_version/src/Game.h

@@ -385,6 +385,8 @@ public:
     int playrx;
     int playry;
     int playgc;
+
+    void quittomenu();
 };
 
 extern Game game;