added clone functions for nodes

Since nodes are are extremely derivative types, we use a clone/copy function
to produce new types of the derived class easily.

app/node/output/timeline/timeline.cpp

@@ -53,6 +53,11 @@ TimelineOutput::TimelineOutput()
   AddParameter(length_output_);
 }
 
+Node *TimelineOutput::copy()
+{
+  return new TimelineOutput();
+}
+
 QString TimelineOutput::Name()
 {
   return tr("Timeline");

app/node/output/timeline/timeline.h

@@ -37,6 +37,8 @@ class TimelineOutput : public Node
 public:
   TimelineOutput();
 
+  virtual Node* copy() override;
+
   virtual QString Name() override;
   virtual QString id() override;
   virtual QString Category() override;

app/node/output/viewer/viewer.cpp

@@ -35,6 +35,11 @@ ViewerOutput::ViewerOutput()
   AddParameter(length_input_);
 }
 
+Node *ViewerOutput::copy()
+{
+  return new ViewerOutput();
+}
+
 QString ViewerOutput::Name()
 {
   return tr("Viewer");

app/node/output/viewer/viewer.h

@@ -36,6 +36,8 @@ class ViewerOutput : public Node
 public:
   ViewerOutput();
 
+  virtual Node* copy() override;
+
   virtual QString Name() override;
   virtual QString id() override;
   virtual QString Category() override;