Fix the width of margins for icons in outline-minor-mode (bug#59719)

Fix the width of margins for icons in outline-minor-mode (bug#59719)

* doc/lispref/display.texi (Icons): Add :width spec.

* lisp/emacs-lisp/icons.el (icons--create): Handle :width as well.

* lisp/outline.el (outline--margin-width, outline-margin-width):
New variables.
(outline-open-in-margins, outline-close-in-margins)
(outline-close-rtl-in-margins): Don't inherit from parents.
Use `:width font' instead of `:height 10'.
(outline-minor-mode): Calculate the number of columns for margins
to fit the icons.