WIP: Reconstruction of background processing update timer,

added unique IDs to the Model / ModelObject / ModelVolume objects,
added a copy of Model hierarchy at the Print,
WIP: new Print::apply() method to update the Print's copy of Model,
to update the Print's PrintObjects, to update status of the Print and
PrintObjects, and to possibly stop the background processing.

xs/xsp/Model.xsp

@@ -262,7 +262,7 @@ ModelMaterial::attributes()
         %code%{ THIS->name = value; %};
     t_model_material_id material_id();
     void set_material_id(t_model_material_id material_id)
-        %code%{ THIS->material_id(material_id); %};
+        %code%{ THIS->set_material_id(material_id); %};
     Ref<ModelMaterial> material();
     
     Ref<DynamicPrintConfig> config()