Move return to lab code to Game::returntolab()

Again, I'll keep gamestate 96 and 97 for compatibility reasons, but
ultimately we shouldn't be using gamestates to return to the lab.

desktop_version/src/Game.h

@@ -380,6 +380,7 @@ public:
     int playgc;
 
     void quittomenu();
+    void returntolab();
 };
 
 extern Game game;