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/node.cpp

@@ -47,6 +47,19 @@ Node::~Node()
   }
 }
 
+void Node::Save(QXmlStreamWriter *writer) const
+{
+  writer->writeStartElement("node");
+
+  writer->writeAttribute("id", id());
+
+  foreach (NodeParam* param, parameters()) {
+    param->Save(writer);
+  }
+
+  writer->writeEndElement(); // node
+}
+
 QString Node::Category() const
 {
   // Return an empty category for any nodes that don't use one