* lisp/outline.el (outline-minor-mode-use-margins): New user option.

* lisp/outline.el (outline-minor-mode-use-margins): New user option.

(outline--use-margins, outline--use-buttons, outline--use-rtl):
New buffer-local internal variables.
(outline-open, outline-close): Move :ascent center to default of define-icon.
Use ASCII-art for text.  Fix docstring and help-echo.
(outline-close-rtl, outline-open-in-margins)
(outline-close-in-margins, outline-close-rtl-in-margins): New icon
definitions.
(outline-minor-mode-highlight-buffer): Remove outline--insert-open-button
since initial outline--fix-up-all-buttons is added now to outline-minor-mode.
(outline-minor-mode): Set buffer-local outline--use-buttons,
outline--use-margins and outline--use-rtl.  Show/hide margins for
outline--use-margins.  Add hook after-change-functions for editable buffers.
Move outline--fix-up-all-buttons for both cases: font-lock and non-font-lock.
(outline--use-buttons-p): Remove function.
(outline--make-button-overlay): Use outline--use-rtl icon outline-close-rtl.
(outline--make-margin-overlay): New function.
(outline--insert-open-button, outline--insert-close-button): Add optional
arg 'use-margins'.
(outline--fix-up-all-buttons): Call outline--insert-close-button
and outline--insert-open-button with arg outline--use-margins.
(outline-cycle-buffer): Remove outline--fix-up-all-buttons
that is already called from outline-flag-region.

* lisp/emacs-lisp/icons.el (icons--create): Handle keywords :rotation
and :ascent with the default value 'center (bug#57813).

* doc/emacs/text.texi (Outline Mode): Mention outline-minor-mode-use-margins.