QmlDesigner: Enable content lib user 3D context menu

Also some relevant tweaks.

Change-Id: I7bace9ce6bd7b45951cc18f7175b4646251196f0
Reviewed-by: Ali Kianian <ali.kianian@qt.io>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>

src/plugins/qmldesigner/components/contentlibrary/contentlibrarymaterialsmodel.cpp

@@ -383,18 +383,4 @@ void ContentLibraryMaterialsModel::removeFromProject(ContentLibraryMaterial *mat
         qWarning() << __FUNCTION__ << err;
 }
 
-bool ContentLibraryMaterialsModel::hasModelSelection() const
-{
-    return m_hasModelSelection;
-}
-
-void ContentLibraryMaterialsModel::setHasModelSelection(bool b)
-{
-    if (b == m_hasModelSelection)
-        return;
-
-    m_hasModelSelection = b;
-    emit hasModelSelectionChanged();
-}
-
 } // namespace QmlDesigner