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

[Stefan Kangas <stefan@marxist.se>]

Severity: wishlist

In M-x customize, we use the etc/images/{un,}checked.xpm and other
icons. These images do not scale when you increase the font, and (this
is subjective but) generally don't look too good.

I think we could get better result if we used Unicode characters like:

- ☑ checked
- ☐ unchecked
- ▼ or ▾ down triangle
- ▶ or ▸ right triangle

I'm less sure about the look of these but they do exist:

- ⬤ or ◉ or 🔘 radio button selected (the last one was added to Unicode
  in 2015 and gives the hex square here)
- ◯ radio button unselected

Obviously how these look depends on the fonts you have installed.  On my
machine, the "Fira Code" font is used, which makes them look very good.
(They also obviously scale when increasing the font size.)

Perhaps we could prefer Unicode icons if they are available, and fall
back on icons if not.  It seems like we can use `char-displayable-p' to
see if there is any font that can display that codepoint.

If we don't think that will produce consistently improved results, we
could also just add a new option to use Unicode characters.

Thoughts?