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

[Pedro Andres Aranda Gutierrez-3]

This patch is a backport of he flat-button style from emacs28

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[Eli Zaretskii]

> From: Pedro Andres Aranda Gutierrez <[hidden email]>
> Date: Mon, 15 Feb 2021 08:16:02 +0100
>
> This patch is a backport of he flat-button style from emacs28

You mean, for the emacs-27 branch?  How serious is the problem, and
how urgent is it to solve it?  my intention was to release Emacs 27.2
_very_ soon.

[Eli Zaretskii]

> From: Pedro Andres Aranda Gutierrez <[hidden email]>
> 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.