bug#56691: outline-minor-mode displays navigation arrow buttons as emojis

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Robert Pluim <rpluim@gmail.com>,  hello@paulwrankin.com,
>   56691@debbugs.gnu.org
> Date: Fri, 29 Jul 2022 15:38:39 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > But the option's value is not a simple scalar, it is a list of
> > preferences, and they are supposed to be used in the order listed when
> > possible.  How do you reconcile that with "I know what I'm doing"?
> 
> If there is only one element in `icon-preference', we could make
> icon-string use that no matter whether we thing Emacs supports it or
> not -- that'd let the user override the machinery completely, which
> might be handy.
> 
> Or have elements like (emoji . force) to force it.

OK, but then the feature becomes much less attractive, IMNSHO.
Basically, we are saying that all the smarts of selecting the best
available method will almost never see action.

And beyond that, there's a problem of how will users know which single
element to put there, unless they only ever work on a single display
type.  Even TTY frames vs GUI frames on the same machine could need
different methods.

So maybe a good compromise is to look at the build details after all,
and consider a system that can display color Emoji as being capable
without checking whether it actually is possible.  That still leaves
the TTY case unsolved, but maybe we can look at the name of the
terminal emulator?  Or have a separate defcustom for what happens on
TTY frames?