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/nodeviewundo.h

@@ -34,7 +34,7 @@ namespace olive {
  */
 class NodeEdgeRemoveCommand : public UndoCommand {
 public:
-  NodeEdgeRemoveCommand(Node* output, NodeInput* input, int element);
+  NodeEdgeRemoveCommand(const NodeOutput& output, const NodeInput& input);
 
   virtual Project* GetRelevantProject() const override;
 
@@ -42,9 +42,8 @@ public:
   virtual void undo() override;
 
 private:
-  Node* output_;
-  NodeInput* input_;
-  int element_;
+  NodeOutput output_;
+  NodeInput input_;
 
 };
 
@@ -55,7 +54,7 @@ private:
  */
 class NodeEdgeAddCommand : public UndoCommand {
 public:
-  NodeEdgeAddCommand(Node* output, NodeInput* input, int element);
+  NodeEdgeAddCommand(const NodeOutput& output, const NodeInput& input);
 
   virtual ~NodeEdgeAddCommand() override;
 
@@ -65,9 +64,8 @@ public:
   virtual void undo() override;
 
 private:
-  Node* output_;
-  NodeInput* input_;
-  int element_;
+  NodeOutput output_;
+  NodeInput input_;
 
   NodeEdgeRemoveCommand* remove_command_;