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.