Add new function to clear tags in tabulated list

* lisp/emacs-lisp/tabulated-list.el (tabulated-list-clear-all-tags):
New function to clear all tags from padding area in current buffer.
* doc/lispref/modes.texi (Tabulated List Mode): Document it.
* etc/NEWS: Announce it.

doc/lispref/modes.texi

@@ -1201,6 +1201,11 @@ width of which is governed by @code{tabulated-list-padding}.
 function advances point by one line.
 @end defun
 
+@defun tabulated-list-clear-all-tags
+This function clears all tags from the padding area in the current
+buffer.
+@end defun
+
 @defun tabulated-list-set-col col desc &optional change-entry-data
 This function changes the tabulated list entry at point, setting
 @var{col} to @var{desc}.  @var{col} is the column number to change, or

etc/NEWS

@@ -1766,6 +1766,11 @@ near the current column in Tabulated Lists (see variables
 list mode: 'w' (which widens the current column) and 'c' which makes
 the current column contract.
 
++++
+*** New function 'tabulated-list-clear-all-tags'.
+This function clears all tags from the padding area in the current
+buffer.  Tags are typically added by calling 'tabulated-list-put-tag'.
+
 ** Text mode
 
 +++

lisp/emacs-lisp/tabulated-list.el

@@ -192,6 +192,19 @@ If ADVANCE is non-nil, move forward by one line afterwards."
   (if advance
       (forward-line)))
 
+(defun tabulated-list-clear-all-tags ()
+  "Clear all tags from the padding area in the current buffer."
+  (unless (> tabulated-list-padding 0)
+    (error "There can be no tags in current buffer"))
+  (save-excursion
+    (goto-char (point-min))
+    (let ((inhibit-read-only t)
+          ;; Match non-space in the first n characters.
+          (re (format "^ \\{0,%d\\}[^ ]" (1- tabulated-list-padding)))
+          (empty (make-string tabulated-list-padding ? )))
+      (while (re-search-forward re nil 'noerror)
+        (tabulated-list-put-tag empty)))))
+
 (defvar tabulated-list-mode-map
   (let ((map (make-sparse-keymap)))
     (set-keymap-parent map (make-composed-keymap