Add "No color" option to "Background Color" and "Font Color"

With CKEditor (default WYSIWYG editor in cycle 16), the font color and background color options are not available anymore. This improvement relies on those features.

Since it didn't receive an update in nearly ten years and IMO this does not sound like something we want today, I'll close this improvement as `Won't fix`.

See https://forum.xwiki.org/t/how-to-change-colour/14759/2 for a recent discussion about the subject of text color.