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

[Stefan Kangas]

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.

Are there any issues with copyright papers here, or is that all sorted?