Quit the application when the user clicks the header bar X

fitnesstrax/app/src/app_window.rs

@@ -56,6 +56,10 @@ impl AppWindow {
             .width_request(800)
             .height_request(746)
             .build();
+        window.connect_destroy({
+            let adw_app = adw_app.clone();
+            move |_| adw_app.quit()
+        });
 
         let stylesheet = String::from_utf8(
             resources_lookup_data(