QmlDesigner: Resolve active scene id also at model attach

When coming back to previously loaded document, the sceneId will be
available at model attach time and there will not be separate changed
notification for it.

Fixes: QDS-8585
Change-Id: Ic8fcd4e2ec9123adc39d0c1cdca3bdb86d3a7924
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/qmldesigner/components/materialbrowser/materialbrowserview.cpp

@@ -229,6 +229,8 @@ void MaterialBrowserView::modelAttached(Model *model)
         refreshModel(true);
         loadPropertyGroups(); // Needs the delay because it uses metaInfo
     });
+
+    m_sceneId = model->active3DSceneId();
 }
 
 void MaterialBrowserView::refreshModel(bool updateImages)