Support tagging.

[RokeJulianLockhart]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.
• I optionally donated to support the Fossify mission.

Feature description

A one-to-many correspondence, possible to combine and exclude when filtering.

I expect it to be implemented how Thunderbird implements IMAP labels – as https://superuser.com/revisions/504078/9 describes.

Why do you want this feature?

To organize notes. Specifically, I tend to organize my Matrix and e-mail messages into task completeness status (due to the absence of CalDAV exposure, as is true for the AOSP SMS/MMS/RCS message store).

Additional information

Previously filed at FossifyOrg/General-Discussion#127 (comment), but refiled at each relevant project's tracker due to each implementation necessarily differing too much.