timeline/project: block serializations to and from xml now include links

Fixes issue where blocks would become unlinked after copy/paste or save/load.

app/node/node.cpp

@@ -82,7 +82,7 @@ void Node::Load(QXmlStreamReader *reader, XMLNodeData& xml_node_data, const QAto
 
       param->Load(reader, xml_node_data, cancelled);
     } else {
-      reader->skipCurrentElement();
+      LoadInternal(reader, xml_node_data);
     }
   }
 }
@@ -99,6 +99,8 @@ void Node::Save(QXmlStreamWriter *writer, const QString &custom_name) const
     param->Save(writer);
   }
 
+  SaveInternal(writer);
+
   writer->writeEndElement(); // node
 }
 
@@ -219,6 +221,15 @@ void Node::DependentEdgeChanged(NodeInput *from)
   }
 }
 
+void Node::LoadInternal(QXmlStreamReader *reader, XMLNodeData &)
+{
+  reader->skipCurrentElement();
+}
+
+void Node::SaveInternal(QXmlStreamWriter *) const
+{
+}
+
 QString Node::ReadFileAsString(const QString &filename)
 {
   QFile f(filename);