bug#46135: 28.0.50; Gnus: icalendar: Buttons in text/calendar view not reachable by TAB

[Jose A. Ortega Ruiz]

When using the excellent gnus-icalendar lib, a set of buttons is
displayed when receiving icalendar mime parts.  E.g.:

      1.  ( ) text/plain          (*) text/calendar       

      [ Show Agenda ]    [ Show Org Entry ]    

      Summary:    Accepted: Let's talk about it in this meeting
      Location:   
      Time:       <2021-02-02 18:00-19:00>
      Organizer:  boss@here.io
      Attendance: Required
      Method:     REPLY

Where [ Show Agenda ] and [ Show Org Entry ] are buttons.  If one goes
to the article buffer and presses TAB, though,  they're skipped and
cannot be navigated that way.  It'd be great i one could :)

Thanks!



In GNU Emacs 28.0.50 (build 15, x86_64-pc-linux-gnu, cairo version 1.16.0)
 of 2021-01-26 built on osgiliath
Repository revision: 8f0a2c84b66ff8d45a9d088a181617417115ec9e
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Debian GNU/Linux bullseye/sid

Configured using:
 'configure --prefix=/usr/local/stow/emacs --with-x-toolkit=no
 --with-imagemagick'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ IMAGEMAGICK
JPEG JSON LCMS2 LIBOTF LIBSELINUX LIBXML2 M17N_FLT MODULES NOTIFY
INOTIFY OLDXMENU PDUMPER PNG RSVG SOUND THREADS TIFF X11 XDBE XIM XPM
ZLIB

Important settings:
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Major mode: Summary

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Load-path shadows:
/home/jao/.emacs.d/elpa.28/telega-20210126.2117/telega hides ~/lib/elisp/telega
/home/jao/.emacs.d/elpa.28/circe-20210111.2141/tracking hides 
/home/jao/.emacs.d/elpa.28/tracking-20201101.1045/tracking
/home/jao/.emacs.d/elpa.28/circe-20210111.2141/shorten hides 
/home/jao/.emacs.d/elpa.28/tracking-20201101.1045/shorten

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-- 
You can't do anything about the length of your life, but you can do
something about its width and depth.
 -H.L. Mencken, writer, editor, and critic (1880-1956)