added save functions to various classes

Each class is responsible for its own saving and loading from the XML data
in order to reduce the complexity of the save code.

app/node/param.cpp

@@ -282,3 +282,23 @@ QByteArray NodeParam::ValueToBytesInternal(const QVariant &v)
 
   return bytes;
 }
+
+void NodeParam::SaveConnections(QXmlStreamWriter *writer) const
+{
+  writer->writeStartElement("connections");
+
+  foreach (NodeEdgePtr edge, edges_) {
+    NodeParam* other;
+
+    if (edge->input() == this) {
+      other = edge->output();
+    } else {
+      other = edge->input();
+    }
+
+    writer->writeTextElement("connection",
+                             QString::number(reinterpret_cast<quintptr>(other)));
+  }
+
+  writer->writeEndElement(); // connections
+}