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/block/block.cpp

@@ -198,6 +198,22 @@ rational Block::MediaToSequenceTime(const rational &media_time) const
   return (media_time - media_in()) / speed() + in();
 }
 
+void Block::LoadInternal(QXmlStreamReader *reader, XMLNodeData &xml_node_data)
+{
+  if (reader->name() == QStringLiteral("link")) {
+    xml_node_data.block_links.append({this, reader->readElementText().toULongLong()});
+  } else {
+    Node::LoadInternal(reader, xml_node_data);
+  }
+}
+
+void Block::SaveInternal(QXmlStreamWriter *writer) const
+{
+  foreach (Block* link, linked_clips_) {
+    writer->writeTextElement(QStringLiteral("link"), QString::number(reinterpret_cast<quintptr>(link)));
+  }
+}
+
 void Block::LengthInputChanged()
 {
   emit LengthChanged(length());