org.el: New option `org-startup-numeroted' and #+startup: num

* lisp/org.el (org-startup-numeroted): New option.
(org-startup-options): New "num" and "nonum" startup keywords.
(org-mode): Startup according `org-startup-numeroted' or the
local #+startup setup.

* doc/org-manual.org (Dynamic Headline Numbering)
(Summary of In-Buffer Settings): Document
`org-startup-numeroted' and #startup: num.

* etc/ORG-NEWS: Document the new startup option.

Thanks to Guillaume MULLER for this idea.

doc/org-manual.org

@@ -18694,6 +18694,11 @@ If ~org-num-skip-footnotes~ is non-~nil~, footnotes sections (see
 You can control how the numbering is displayed by setting
 ~org-num-face~ and ~org-num-format-function~.
 
+#+vindex: org-startup-numeroted
+You can also turn this mode globally for all Org files by setting the
+option ~org-startup-numeroted~ to =t=, or locally on a file by using
+=#+startup: num=.
+
 ** The Very Busy {{{kbd(C-c C-c)}}} Key
 :PROPERTIES:
 :DESCRIPTION: When in doubt, press @kbd{C-c C-c}.
@@ -18865,6 +18870,13 @@ changes.
   | =indent=   | Start with Org Indent mode turned on.  |
   | =noindent= | Start with Org Indent mode turned off. |
 
+  #+vindex: org-startup-numeroted
+  Dynamic virtual numerotation of headlines is controlled by the variable
+  ~org-startup-numeroted~.
+
+  | =num=   | Start with Org num mode turned on.  |
+  | =nonum= | Start with Org num mode turned off. |
+
   #+vindex: org-startup-align-all-tables
   Aligns tables consistently upon visiting a file.  The
   corresponding variable is ~org-startup-align-all-tables~ with

etc/ORG-NEWS

@@ -116,6 +116,12 @@ minutes.
 You can now use =C-x C-+= in column view: the columns face size will
 increase or decrease, together with the column header size.
 
+*** New startup option =#+startup: num=
+
+When this startup option is set, display headings as numeroted.
+
+Use =#+startup: nonum= to turn this off.
+
 *** =ob-clojure.el= supports inf-clojure.el and ClojureScript evaluation
 
 You can now set ~(setq org-babel-clojure-backend 'inf-clojure)~ and
@@ -203,6 +209,14 @@ This option will add a timeout to notifications.
 
 Functions in this hook are run after ~org-agenda-filter~ is called.
 
+*** New option ~org-startup-numeroted~
+
+When this option is =t=, Org files will start using ~(org-num-mode 1)~
+and headings will be visually numeroted.
+
+You can turn this on/off on a per-file basis with =#+startup: num= or
+=#+startup: nonum=.
+
 ** Removed or renamed functions and variables
 
 *** Renamed ~org-columns-set-tags-or-toggle~

lisp/org.el

@@ -972,6 +972,19 @@ the following lines anywhere in the buffer:
 	  (const :tag "Not" nil)
 	  (const :tag "Globally (slow on startup in large files)" t)))
 
+(defcustom org-startup-numeroted nil
+  "Non-nil means turn on `org-num-mode' on startup.
+This can also be configured on a per-file basis by adding one of
+the following lines anywhere in the buffer:
+
+   #+STARTUP: num
+   #+STARTUP: nonum"
+  :group 'org-structure
+  :package-version '(Org . "9.4")
+  :type '(choice
+	  (const :tag "Not" nil)
+	  (const :tag "Globally" t)))
+
 (defcustom org-use-sub-superscripts t
   "Non-nil means interpret \"_\" and \"^\" for display.
 
@@ -4050,6 +4063,8 @@ After a match, the following groups carry important information:
     ("content" org-startup-folded content)
     ("indent" org-startup-indented t)
     ("noindent" org-startup-indented nil)
+    ("num" org-startup-numeroted t)
+    ("nonum" org-startup-numeroted nil)
     ("hidestars" org-hide-leading-stars t)
     ("showstars" org-hide-leading-stars nil)
     ("odd" org-odd-levels-only t)
@@ -4809,6 +4824,7 @@ The following commands are available:
      (when org-startup-with-latex-preview (org-latex-preview '(16)))
      (unless org-inhibit-startup-visibility-stuff (org-set-startup-visibility))
      (when org-startup-truncated (setq truncate-lines t))
+     (when org-startup-numeroted (require 'org-num) (org-num-mode 1))
      (when org-startup-indented (require 'org-indent) (org-indent-mode 1))))
 
   ;; Activate `org-table-header-line-mode'