* lisp/buff-menu.el (Buffer-menu-group-sort-by): New defcustom.

* lisp/buff-menu.el (Buffer-menu-group-sort-by): New defcustom.

(list-buffers--refresh): Use Buffer-menu-group-sort-by instead of the
hard-coded function (bug#70150).
(Buffer-menu-group-sort-alphabetically): New function as an option for
'Buffer-menu-group-sort-by'.
(list-buffers-noselect): Remove setting	outline-minor-mode-use-buttons
to 'in-margins' that it not required for this feature to work correctly.

* lisp/emacs-lisp/tabulated-list.el (tabulated-list-groups-sort):
Add optional argument 'level'.