bug#46528: 27.1.91; Backport flat-button from emacs28

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Sat, 3 Apr 2021 19:45:30 -0500
> Cc: Pedro Andres Aranda Gutierrez <paaguti@gmail.com>, 46528@debbugs.gnu.org
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Pedro Andres Aranda Gutierrez <paaguti@gmail.com>
> >> Date: Mon, 15 Feb 2021 16:42:36 +0100
> >>
> >> It could go with 27.3 if you prefer.
> >
> > No problem here, but please note that there's no guarantee there will
> > be Emacs 27.3.  We could decide to release Emacs 28.1 as the next
> > version.
> 
> I suppose we could add this to the emacs-27 branch now that Emacs 27.2
> is released.

Actually, I'd prefer not to, upon further thinking: this is a new
feature, and we don't add features to the release branch.  (I thought
I wrote that already?)

Sorry, I must have paid insufficient attention when I wrote the above
back then.