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");