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();