nodes: link edges to IDs rather than objects

Allows code to know whether the connected parameters has been removed or
not.

app/node/edge.h

@@ -22,13 +22,16 @@
 #define EDGE_H
 
 #include <memory>
+#include <QString>
 
 #include "common/define.h"
 
 OLIVE_NAMESPACE_ENTER
 
-class NodeOutput;
+class Node;
 class NodeInput;
+class NodeOutput;
+class NodeParam;
 
 /**
  * @brief A connection between two node parameters (a NodeOutput and a NodeInput)
@@ -44,19 +47,37 @@ public:
    */
   NodeEdge(NodeOutput* output, NodeInput* input);
 
+  Node* output_node() const
+  {
+    return output_.node;
+  }
+
+  Node* input_node() const
+  {
+    return input_.node;
+  }
+
   /**
    * @brief Return the output parameter this edge is connected to
    */
-  NodeOutput* output();
+  NodeOutput* output() const;
 
   /**
    * @brief Return the input parameter this edge is connected to
    */
-  NodeInput* input();
+  NodeInput* input() const;
 
 private:
-  NodeOutput* output_;
-  NodeInput* input_;
+  struct Connection {
+    Node* node;
+    QString id;
+  };
+
+  static Connection ParamToConnection(NodeParam* param);
+
+  Connection output_;
+  Connection input_;
+
 };
 
 using NodeEdgePtr = std::shared_ptr<NodeEdge>;