[examples] test_window_impl: Add "Close" button to stacked modals

The "Close" button was forgotten in previous commits.

imgui-examples/examples/test_window_impl.rs

@@ -697,6 +697,10 @@ CTRL+click on individual component to input value.\n",
                             ui.close_current_popup();
                         }
                     });
+
+                    if ui.button(im_str!("Close"), (0.0, 0.0)) {
+                        ui.close_current_popup();
+                    }
                 });
             });
         }