Show which colors have low contrast

[egmontkob]

Hook up to WCAG's formula for each color role.

Show somewhere somehow (could we do it next to the skin definition view, matching line-by-line, scrolling together?? ) for each color pair definition the WCAG score, painted in e.g. green/yellow/red according to their A/AA/AAA match.

(I've thrown in keywords that I'm not familiar with either, so first let's learn more about this topic.)

(Inspired by MidnightCommander/mc#4775.)

[egmontkob]

mc-show-skin-contrast.patch

This is a demo patch against mc (its current master at 4ad919a05), printing the contrast values to stderr (redirect that file descriptor to a file or another terminal).

This gives a rough idea about how "good" a skin is, regarding its contrasts.

The patch is not meant to be upstreamed and I won't continue working on it; it should be replaced by a similar solution in the online skin viewer/editor.