bug#56779: 29.0.50; [wishlist] Function to check if a mouse-button is pressed

[Visuwesh]

Currently, to know if a mouse button is being pressed, one has to keep
track of state by cleverly utilising [down-mouse-N] and [mouse-N] events
(see [1]).  It is certainly no easy task, it would be really nice if
Emacs had the equivalent of `xinput --query-state <mouse-id>' [2]
(the implementation of which can be found in state.c in xinput's
source).

The reason I ask this is because I would really like to have mouse
chording in Emacs but currently it is too hard to implement it (BTW,
mouse-copy.el talks about mouse-chording).

    1. https://github.com/akrito/acme-mouse
    2. All you need to do is,
           % xinput --query-state <mouse-id> |sed -n 's/[ \t]\+button\[1\]=//p'
       to get the state of mouse-1.

In GNU Emacs 29.0.50 (build 26, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2022-07-22 built on astatine
Repository revision: 12a3137cd381cb743768033e789b900b015041d7
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101003
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure --with-sound=alsa --with-x-toolkit=lucid --with-json
 --without-xaw3d --without-gconf --without-libsystemd --without-cairo'
Configured features:
ACL DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG JSON
LIBOTF LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS WEBP X11 XDBE XFT
XIM XINPUT2 XPM LUCID ZLIB
Important settings:
  value of $LC_MONETARY: ta_IN.UTF-8
  value of $LC_NUMERIC: ta_IN.UTF-8
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
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  shell-dirtrack-mode: t
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  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  use-hard-newlines: t
  tab-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  undelete-frame-mode: t
  transient-mark-mode: (only . t)
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  abbrev-mode: t

Load-path shadows:
/home/viz/lib/emacs/straight/build/faceup/faceup hides 
/home/viz/lib/ports/emacs/lisp/emacs-lisp/faceup

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Memory information:
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 (string-bytes 1
               6737836) (vectors 16 86388) (vector-slots 8 1108290 50611) 
(floats 8 711
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 (
  intervals ?8 10835 1580) (buffers 992 40))