nodes: link edges to IDs rather than objects

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

app/node/output/viewer/viewer.cpp

@@ -62,6 +62,11 @@ ViewerOutput::ViewerOutput() :
   uuid_ = QUuid::createUuid();
 }
 
+ViewerOutput::~ViewerOutput()
+{
+  DisconnectAll();
+}
+
 Node *ViewerOutput::copy() const
 {
   return new ViewerOutput();

app/node/output/viewer/viewer.h

@@ -47,6 +47,8 @@ class ViewerOutput : public Node
 public:
   ViewerOutput();
 
+  virtual ~ViewerOutput() override;
+
   virtual Node* copy() const override;
 
   virtual QString Name() const override;