reduce SessionModel/SessionItem usage

Mainly refactoring to migrate SessionModel/SessionItem (reduce exposure of SessionXxx in classes, reduce usage of SessionXxx functionality)

GUI/Model/Material/MaterialItemUtils.cpp

@@ -37,7 +37,7 @@ QString GUI::MaterialUtil::defaultMaterialIdentifier()
     if (!GUI::Model::MaterialModelStore::materialModel())
         return QString();
 
-    auto materials = GUI::Model::MaterialModelStore::materialModel()->topItems<MaterialItem>();
+    const auto materials = GUI::Model::MaterialModelStore::materialModel()->materialItems();
     return materials.isEmpty() ? QString() : materials.front()->identifier();
 }