private void separatorToolStripMenuItem_Click(object sender, EventArgs e)
        {
            var cnt = this.SelectedContainer;

            if (cnt != null)
            {
                //at end
                var item = _flexLayout.CreateSeparator();
                cnt.Insert(item, cnt.ItemCount);
                this.SelectedNode.Nodes.Add(CreateNode(item));
            }
            else
            {
                var menu = this.SelectedNode.Tag as IMenu;
                if (menu != null)
                {
                    //at end
                    var item = _flexLayout.CreateSeparator();
                    menu.Insert(item, menu.ItemCount);
                    this.SelectedNode.Nodes.Add(CreateNode(item));
                }
            }
        }