use strings to connect nodes

This makes the node system somewhat more high-level with the intent of making
working with them far more flexible and stable. By making the architecture more
abstracted, it becomes far less rigid which should allow us to do even more
with it and make it much less crash prone.

app/widget/nodeview/nodeviewedge.h

@@ -39,18 +39,18 @@ class NodeViewItem;
 class NodeViewEdge : public QGraphicsPathItem
 {
 public:
-  NodeViewEdge(Node* output, NodeInput *input, int element,
+  NodeViewEdge(const NodeOutput& output, const NodeInput& input,
                NodeViewItem* from_item, NodeViewItem* to_item,
                QGraphicsItem* parent = nullptr);
 
   NodeViewEdge(QGraphicsItem* parent = nullptr);
 
-  Node* output() const
+  const NodeOutput& output() const
   {
     return output_;
   }
 
-  NodeInput* input() const
+  const NodeInput& input() const
   {
     return input_;
   }
@@ -112,9 +112,9 @@ protected:
 private:
   void Init();
 
-  Node* output_;
+  NodeOutput output_;
 
-  NodeInput* input_;
+  NodeInput input_;
 
   int element_;