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.h

@@ -25,6 +25,7 @@
 #include <QMutex>
 #include <QObject>
 #include <QPointF>
+#include <QXmlStreamWriter>
 
 #include "common/rational.h"
 #include "node/dependency.h"
@@ -64,6 +65,11 @@ public:
    */
   virtual Node* copy() const = 0;
 
+  /**
+   * @brief Save this node into a text/XML format
+   */
+  void Save(QXmlStreamWriter* writer) const;
+
   /**
    * @brief Return the name of the node
    *