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/widget/nodeview/nodeviewitem.h

@@ -82,6 +82,8 @@ protected:
   virtual void mouseMoveEvent(QGraphicsSceneMouseEvent *event) override;
   virtual void mouseReleaseEvent(QGraphicsSceneMouseEvent *event) override;
 
+  virtual QVariant itemChange(QGraphicsItem::GraphicsItemChange change, const QVariant &value) override;
+
 private:
   /**
    * @brief Get the relative position to draw text for a parameter at a certain index