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

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Sat, 13 Mar 2021 05:51:26 -0600
> Cc: alan@idiocy.org, 47074@debbugs.gnu.org
> 
> I mostly placed them there as the old names were to my mind not ideal,
> and I hoped we could have something better/more consistent.
> 
> Here's a comparison:
> 
>     OLD NAME          NEW NAME
>     ---------------   ---------------
>     checked.xpm       ui/checkbox.svg
>     unchecked.xpm     ui/checkbox-checked.svg
>     N/A               ui/checkbox-mixed.svg
>     N/A               ui/radio-checked.svg
>     N/A               ui/radio-mixed.svg
>     N/A               ui/radio.svg
>     custom/down.xpm   ui/triangle-down.svg   [not yet added]
>     N/A               ui/triangle-left.svg   [not yet added]
>     custom/right.xpm  ui/triangle-right.svg  [not yet added]
>     N/A               ui/triangle-up.svg     [not yet added]

I don't think names matter in this case, as no one sees them except
the source code.