implemented clear() function for nodegraphs

Added a convenient (destructive) way of emptying a NodeGraph completely.

app/node/graph.cpp

@@ -25,6 +25,14 @@ NodeGraph::NodeGraph()
 
 }
 
+void NodeGraph::Clear()
+{
+  foreach (Node* node, node_children_) {
+    delete node;
+  }
+  node_children_.clear();
+}
+
 void NodeGraph::AddNode(Node *node)
 {
   if (ContainsNode(node)) {
@@ -91,7 +99,7 @@ QList<Node *> NodeGraph::TakeNodeWithItsDependencies(Node *node, QObject *new_pa
   return deps;
 }
 
-QList<Node *> NodeGraph::nodes()
+const QList<Node *> &NodeGraph::nodes()
 {
   return node_children_;
 }

app/node/graph.h

@@ -37,6 +37,11 @@ public:
    */
   NodeGraph();
 
+  /**
+   * @brief Destructively destroys all nodes in the graph
+   */
+  void Clear();
+
   /**
    * @brief Add a node to this graph
    *
@@ -73,7 +78,7 @@ public:
   /**
    * @brief Retrieve a complete list of the nodes belonging to this graph
    */
-  QList<Node*> nodes();
+  const QList<Node*>& nodes();
 
   /**
    * @brief Returns whether a certain Node is in the graph or not