* lisp/outline.el: Pre-compute some frequent data for button icons (bug#57813)

* lisp/outline.el: Pre-compute some frequent data for button icons (bug#57813)

(outline--button-icons): New buffer-local variable.
(outline-minor-mode): Set outline--button-icons.
Unify overlay name 'outline-margin' with 'outline-button'.
(outline--make-button-overlay, outline--make-margin-overlay)
(outline--insert-open-button, outline--insert-close-button): Remove functions.
(outline--create-button-icons, outline--insert-button): New functions
with code refactored from old functions.  Add more support for icon faces.
(outline--fix-up-all-buttons): Use outline--insert-button.
(outline--fix-buttons-after-change): Unify overlay name
'outline-margin' with 'outline-button'.

* lisp/minibuffer.el (completions-group-separator): Change face
attribute :strike-through to :underline.