def del_child(child)
      @children.delete(child)
      @@registry.delete(child)
      child.finalize!
      child.win.delwin if !!child.win
    end
def del_child(child)
      @children.delete(child)
      @@registry.delete(child)
      child.finalize!
      child.win.delwin if !!child.win
    end
Remove child window, like #create_child, this is used internally
 and should not be invoked by the end user