public void MoveDown()
{
if (SelectedColumns.Count == 0 || SelectedColumns.Last().IsSelected)
{
return;
}
var selected = SelectedColumns.Where(c => c.IsSelected).Reverse().ToArray();
foreach (var item in selected)
{
var index = SelectedColumns.IndexOf(item);
SelectedColumns.RemoveAt(index);
SelectedColumns.Insert(index + 1, item);
}
}