use uuid for identifying viewers

To aid generating a cache ID, each viewer node needs a UUID. Previous iterations
tried to hash a name and time, but a UUID is a much more efficient way to do
this.

app/node/output/viewer/viewer.cpp

@@ -33,6 +33,9 @@ ViewerOutput::ViewerOutput()
   length_input_ = new NodeInput("length_in");
   length_input_->set_data_type(NodeInput::kRational);
   AddInput(length_input_);
+
+  // Create UUID for this node
+  uuid_ = QUuid::createUuid();
 }
 
 Node *ViewerOutput::copy() const
@@ -125,6 +128,11 @@ rational ViewerOutput::Length()
   return 0;
 }
 
+const QUuid &ViewerOutput::uuid() const
+{
+  return uuid_;
+}
+
 void ViewerOutput::DependentEdgeChanged(NodeInput *from)
 {
   if (from == texture_input_) {