public void AddToList(IDockContent content)
            {
                if (ListContent.Contains(content) || IsInActiveList(content))
                {
                    return;
                }

                ListContent.Add(content);
            }
 public void RemoveFromList(IDockContent content)
 {
     if (IsInActiveList(content))
     {
         RemoveFromActiveList(content);
     }
     if (ListContent.Contains(content))
     {
         ListContent.Remove(content);
     }
 }