enable titlebar feature

src/app/browser.rs

@@ -9,7 +9,7 @@ pub fn new(app: &Application) -> ApplicationWindow
     return ApplicationWindow::builder().application(app)
                                        .default_width(640)
                                        .default_height(480)
-                                     //.titlebar(&header::new())
+                                       .titlebar(&header::new())
                                        .child(&main::new())
                                        .build();
 }