Customize tabulated-list sort indicators

Customize tabulated-list sort indicators

This allows the user to customize the sorting indicators displayed
near the current column.

* lisp/emacs-lisp/tabulated-list.el (tabulated-list): New group.
(tabulated-list-gui-sort-indicator-asc)
(tabulated-list-gui-sort-indicator-desc)
(tabulated-list-tty-sort-indicator-asc)
(tabulated-list-tty-sort-indicator-desc): New defcustomd.
(tabulated-list-glyphless-char-display): Remove.
(tabulated-list-make-glyphless-char-display-table): New function.

* doc/lispref/modes.texi (Tabulated List Mode): Add documentation
for new options.

* etc/NEWS: Mention the new options.