bug#9402: list-colors-display

[Juri Linkov]

>> 2. New faces `gtk_selection_fg_color' and `gtk_selection_bg_color'
>> now are available for using on GTK but they are not listed in
>> `list-colors-display', and they can't be selected and completed in
>> `customize-face'.
>
> Did this ever work?  Emacs always had platform-specific colors besides
> those in rgb.txt, I don't think this is something new.

Yes, it never worked.

>> There are more colors that are valid for using but hidden from users
>> such as e.g. on a tty: `unspecified-fg', `unspecified-bg'.
>
> They are meaningless and cannot be set by Emacs.  IOW, they are
> "read-only", as far as Emacs is concerned.  Exposing them to users
> would be confusing.

All colors are "read-only" for users in the sense that users can't
change them.  But platform-specific colors are not different from other
colors in the sense that they are used in face customization.

IOW, when the user types `M-x customize-face RET region RET'
then s?he sees the color name "gtk_selection_fg_color".
Then the user might change it to some other color, but after that
it's difficult to restore the original default color name,
because the user don't remember this color name literally
and can't select it using completion or the "Choose" button
that lists all color names.