Use icons in outline

Use icons in outline

* lisp/help.el (describe-bindings): Don't force buttons on
(bug#56691).

* lisp/outline.el (outline-minor-mode-use-buttons): Default
buttons on in special-mode buffers.
* lisp/outline.el (outline-minor-mode-buttons): Remove.
(outline-open, outline-close): New icons.
(outline-minor-mode-highlight-buffer): Use the new predicate to
switch on/off.
(outline--make-button): Remove.
(outline--make-button-overlay): Use icons.el instantiation.
(outline--valid-emoji-p): Remove.
(outline--valid-char-p): Remove.
(outline--insert-open-button, outline--insert-close-button): Make
point movement better in *Help* buffers.