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/graph.cpp

@@ -72,12 +72,12 @@ void NodeGraph::TakeNode(Node *node, QObject* new_parent)
   emit NodeRemoved(node);
 }
 
-const QList<Node *> &NodeGraph::nodes()
+const QList<Node *> &NodeGraph::nodes() const
 {
   return node_children_;
 }
 
-bool NodeGraph::ContainsNode(Node *n)
+bool NodeGraph::ContainsNode(Node *n) const
 {
   return (n->parent() == this);
 }