bug#43397: 28.0.50; Adding tool bar items: update tool bar

[Caio Henrique]

Gnus has a function called gnus-tool-bar-update. I tried using it but
got the same result, i.e. the icon is only displayed when I click with
the mouse on the buffer. 

1. emacs -Q
2. eval this:
(progn
  (defvar tool-bar-mode)

  (defun gnus-tool-bar-update (&rest ignore)
    "Update the tool bar."
    (when (and (boundp 'tool-bar-mode)
               tool-bar-mode)
      (let* ((args nil)
             (func (cond ((fboundp 'tool-bar-update)
                          'tool-bar-update)
                         ((fboundp 'force-window-update)
                          'force-window-update)
                         ((fboundp 'redraw-frame)
                          (setq args (list (selected-frame)))
                          'redraw-frame)
                         (t 'ignore))))
        (apply func args))))

  (tool-bar-add-item-from-menu 'undo-redo
                               "redo" nil :vert-only t)

  (gnus-tool-bar-update))