Material refactoring

• reduce SessionModel exposure
• reduce SessionModel usage
• avoid global access to material model
• minor changes

GUI/Model/Material/MaterialModel.cpp

@@ -50,6 +50,11 @@ MaterialModel::~MaterialModel()
         GUI::Model::MaterialModelStore::unsubscribe(this);
 }
 
+void MaterialModel::clear() 
+{
+    SessionModel::clear();
+}
+
 MaterialModel* MaterialModel::createCopy(SessionItem* parent)
 {
     auto* result = new MaterialModel();