messageTray: Let the tray decide whether to show a banner

Always use the same code path to add new messages to a source, and
let the `MessageTray` decide whether it shows a banner.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3173>

js/ui/extensionSystem.js

@@ -354,7 +354,7 @@ export class ExtensionManager extends Signals.EventEmitter {
                 _('Extension updates are ready to be installed.'));
             notification.connect('activated',
                 () => source.open());
-            source.showNotification(notification);
+            source.addNotification(notification);
         }
     }