QmlDesigner: Reset puppet when material is added

This is workaround for material rendering issues in quick3d when a
material is shared by multiple windows.

Fixes: QDS-7096
Fixes: QDS-7118
Change-Id: I64fe2e51c5dda8e238502e5d926b6938b7b0731a
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

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

@@ -120,6 +120,11 @@ void MaterialBrowserModel::setHasModelSelection(bool b)
     emit hasModelSelectionChanged();
 }
 
+QList<ModelNode> MaterialBrowserModel::materials() const
+{
+    return m_materialList;
+}
+
 void MaterialBrowserModel::setSearchText(const QString &searchText)
 {
     QString lowerSearchText = searchText.toLower();