[orca-list] The use of color names is now configurable via GUI

[Joanmarie Diggs <jdiggs igalia com>]

Hi again.

As you all know, I added feature to speak color names instead of RGB
values a few release cycles ago. My assumption was (and for the most
part still is) that very, very few people would want RGB values rather
than color name so turning this feature off could only be done via
orca-customizations. But given that I'm GUI-fying all the things, here's
another one for your testing pleasure.

Thanks!
--joanie