Allow multi-level outlines in tabulated-list-groups used by list-buffers

Allow multi-level outlines in tabulated-list-groups used by list-buffers

* lisp/emacs-lisp/tabulated-list.el (tabulated-list-groups)
(tabulated-list-groups-categorize, tabulated-list-groups-sort)
(tabulated-list-groups-flatten): New functions (bug#70150).

* lisp/buff-menu.el (Buffer-menu-group-by): Change type from a function
to a list of functions.
(list-buffers--refresh): Use the function 'tabulated-list-groups' where
:path-function uses a list of functions from 'Buffer-menu-group-by', and
:sort-function is hard-coded to sort groups by name.
(Buffer-menu-group-by-mode, Buffer-menu-group-by-root): Remove prefix "*".