feat: Add quit callback support to engine and UI components

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

apps/wolf_3d_gui/lib/screens/game_screen.dart

@@ -45,6 +45,9 @@ class _GameScreenState extends State<GameScreen> {
         widget.wolf3d.clearActiveDifficulty();
         Navigator.of(context).pop();
       },
+      onQuit: () {
+        SystemNavigator.pop();
+      },
     );
   }