Add support colorized categories

On some cases I need to give special colors to categories prior to addressbook's color (for example "mailing list" - because mails to such mailing lists will be delivered to many subscribers). These changes add ability to set color for each category. I'm happy if this feature is supported by the upstream. How about it?

Note that this merge request may conflict to !405 (closed) . I'll update another if one of these MR is merged.