* lisp/outline.el: More improvements for buttons/margins (bug#57813)

* lisp/outline.el: More improvements for buttons/margins (bug#57813)

(outline-minor-mode-use-margins): Don't use for modes derived from help-mode
that are handled by 'outline-minor-mode-use-buttons'.
(outline-open, outline-close, outline-close-rtl): Use image height
proportional to font height of 0.8em.
(outline-minor-mode): Remove overlays 'outline-button' and 'outline-margin',
and move such overlay removal after the call of 'outline-show-all'
that might trigger overlay addition.