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.h

@@ -25,6 +25,7 @@
 #include <QObject>
 #include <QVariant>
 #include <QVector>
+#include <QXmlStreamWriter>
 
 #include "common/rational.h"
 #include "node/edge.h"
@@ -229,6 +230,11 @@ public:
 
   virtual ~NodeParam() override;
 
+  /**
+   * @brief Save function
+   */
+  virtual void Save(QXmlStreamWriter* writer) const = 0;
+
   /**
    * @brief Return ID of this parameter
    */
@@ -368,6 +374,8 @@ signals:
   void EdgeRemoved(NodeEdgePtr edge);
 
 protected:
+  void SaveConnections(QXmlStreamWriter* writer) const;
+
   /**
    * @brief Internal list of edges
    */