moved timeline undoable commands to using the new NodeAddCommand

Previous iteration used some "magic code" that added clips automatically to the
timeline. This was functional but ultimately outside of the undo commands'
control meaning nodes could be infinitely added and abandoned. This makes the
add process part of the undo command which means it's all undoable as the user
would expect.

app/node/node.cpp

@@ -468,6 +468,16 @@ QVariant Node::InputValueFromTable(NodeInput *input, const NodeValueTable &table
   return table.Get(find_data_type);
 }
 
+const QPointF &Node::GetPosition()
+{
+  return position_;
+}
+
+void Node::SetPosition(const QPointF &pos)
+{
+  position_ = pos;
+}
+
 void Node::AddInput(NodeInput *input)
 {
   AddParameter(input);