private void cutNodesToolStripMenuItem_Click(object sender, EventArgs e)
{
List<ElementNode> cutNodes = new List<ElementNode>();
foreach (TreeNode treenode in multiSelectTreeviewElementsGroups.SelectedNodes) {
cutNodes.Add(treenode.Tag as ElementNode);
DeleteNode(treenode);
}
_clipboardNodes = cutNodes;
PopulateNodeTree();
}