dbusServices/extensions: Stop using GtkContainer API

The interface has been removed in GTK4, so use widget-specific API
for adding children where possible.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1495>

js/dbusServices/extensions/extensionsService.js

@@ -204,7 +204,7 @@ var ExtensionPrefsDialog = GObject.registerClass({
             prefsModule.init(extension.metadata);
 
             const widget = prefsModule.buildPrefsWidget();
-            this._stack.add(widget);
+            this._stack.add_named(widget, 'prefs');
             this._stack.visible_child = widget;
         } catch (e) {
             this._setError(e);