bug#71216: 29.3; MH-E: want to sort threads by most recent message

[Mike Kupfer <kupfer@rawbw.com>]

(This came up recently in the mh-e-users mailing list on SourceForge.
I'm opening this bug to track the request.)

When MH-E displays a folder in threaded mode, the threads appears to be
sorted according to the oldest message in the thread.  I write "appears
to be" because the code is a little hard to follow.  I think it's using
each message's MH message number in the folder as a proxy for the
message's age.

The request is to provide a way to toggle between that and a sort order
where the thread's position relative to other threads is determined by
the newest message in the thread, rather than the oldest.

Example:

current sorting:

1 01/01 [Alice] subject1
3 01/02   [Bob] re: subject1
5 01/05     [Alice] re: subject1
2 01/02 [Alice] subject2
4 01/03   [Bob] re: subject2

alternate sorting:

2 01/02 [Alice] subject2
4 01/03   [Bob] re: subject2
1 01/01 [Alice] subject1
3 01/02   [Bob] re: subject1
5 01/05     [Alice] re: subject1

In GNU Emacs 29.3 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.16.0, Xaw scroll bars) of 2024-03-25 built on alto
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --prefix=/usr/local'

Configured features:
CAIRO FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG LIBSELINUX
LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM LUCID ZLIB

Important settings:
  value of $LC_TIME: C
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: MH-Show

Minor modes in effect:
  server-mode: t
  shell-dirtrack-mode: t
  delete-selection-mode: t
  display-time-mode: t
  global-eldoc-mode: t
  show-paren-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  buffer-read-only: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow mh-identity emacsbug mh-funcs sort gnus-async gnus-bcklg
gnus-kill gnus-ml disp-table url-http url-gw url-cache url-auth
url-handlers nnrss nndoc nndraft network-stream nsm gnus-agent gnus-srvr
nnvirtual nntp gnus-cache misearch multi-isearch sgml-mode facemenu
dired-aux dired-x crm vc-hg diff-mode easy-mmode cal-move diary-lib
diary-loaddefs cal-iso oc-basic ol-eww eww url-queue mm-url ol-rmail
ol-mhe ol-irc ol-info ol-gnus nnselect ol-docview doc-view filenotify
jka-compr image-mode exif ol-bibtex bibtex ol-bbdb ol-w3m ol-doi
org-link-doi cl-extra help-mode face-remap org-agenda org-element
org-persist xdg org-id avl-tree generator org-refile org ob ob-tangle
ob-ref ob-lob ob-table ob-exp org-macro org-src ob-comint org-pcomplete
org-list org-footnote org-faces org-entities ob-emacs-lisp ob-core
ob-eval org-cycle org-table org-keys oc org-loaddefs find-func cal-menu
calendar cal-loaddefs ol rx org-fold org-fold-core org-compat
org-version org-macs format-spec mh-alias textsec uni-scripts
idna-mapping ucs-normalize uni-confusable textsec-check shr-color color
qp mm-archive mail-extr mh-mime pp mh-search mh-letter mh-show goto-addr
thingatpt gnus-cite mule-util mh-thread mh-inc hl-line mh-tool-bar
mh-acros mh-seq mh-xface mh-utils mh-folder which-func imenu files-x
mdk-mail gnus-mh gnus-msg mh-comp mh-scan mh-gnus gnus-dup nnmh
gnus-score score-mode gnus-art mm-uu mml2015 mm-view mml-smime smime
gnutls dig gnus-sum shr pixel-fill kinsoku url-file svg dom browse-url
url url-proxy url-privacy url-expand url-methods url-history url-cookie
generate-lisp-file url-domsuf url-util url-parse auth-source cl-seq
eieio eieio-core cl-macs json map gv url-vars gnus-group gnus-undo
gnus-start gnus-dbus dbus xml gnus-cloud nnimap nnmail mail-source utf7
nnoo parse-time iso8601 gnus-spec gnus-int gnus-range gnus-win gnus
nnheader range wid-edit mh-e mh-buffers mh-loaddefs message sendmail
mailcap yank-media puny dired dired-loaddefs rfc822 mml mml-sec
password-cache epa derived epg rfc6068 epg-config gnus-util
text-property-search time-date mm-decode mm-bodies mm-encode mail-parse
rfc2231 rfc2047 rfc2045 mm-util ietf-drums mail-prsvr mailabbrev
mail-utils gmm-utils mailheader server noutline outline icons cc-mode
cc-fonts cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars
cc-defs byte-opt bytecomp byte-compile shell subr-x pcomplete comint
ansi-osc ansi-color ring xcscope advice delsel time vc vc-dispatcher
timeclock cl-loaddefs cl-lib mdk-hacks rmc iso-transl tooltip cconv
eldoc paren electric uniquify ediff-hook vc-hooks lisp-float-type
elisp-mode mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd
fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow
isearch easymenu timer select scroll-bar mouse jit-lock font-lock syntax
font-core term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads inotify dynamic-setting
system-font-setting font-render-setting cairo x-toolkit x multi-tty
make-network-process emacs)

Memory information:
((conses 16 526387 77946)
 (symbols 48 32725 5)
 (strings 32 132303 11351)
 (string-bytes 1 3848560)
 (vectors 16 87468)
 (vector-slots 8 1893799 183057)
 (floats 8 572 542)
 (intervals 56 5512 2316)
 (buffers 976 64))