bug#47074: Replace XPM icons with Unicode codepoints in customize/widgets

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Thu, 11 Mar 2021 18:01:06 +0100
> Cc: 47074@debbugs.gnu.org
> 
> > - ⬤ or ◉ or 🔘 radio button selected (the last one was added to Unicode
> >   in 2015 and gives the hex square here)
> > - ◯ radio button unselected
> 
> These look like this here:

With Symbola as the font (which is what we have by default), the
result is much worse than images.