Exit on exit

src/game.js

@@ -28,7 +28,9 @@ class Game {
             if (this.mainScreen.after === AfterMenu.Start) {
                 this.changeScreen(this.scene);
             } else if (this.mainScreen.after === AfterMenu.Exit) {
-                alert("I don't know how to exit");
+                if (navigator.app !== undefined && typeof navigator.app.exitApp === 'function') {
+                    navigator.app.exitApp();
+                }
             }
         }
     }