Susimail: New mail should remain flagged as new until opened

[Reportage]

Currently new mail is only flagged for the current session. Unopened mail should remain as new (and flagged) until opened.

[zzz]

Yup. Unfortunately we don't currently have any place to save per-email state across sessions.

We also don't currently allow users to click on the new or spam flag (or empty column) to change it. I think that was an echelon request. We would have to have a place to store the status if we did.

Thunderbird seems to add (or did) some flags to the file itself, as headers, including X-Mozilla-Keys, X-UIDL, X-Account-Key, X-Mozilla-Status, and X-Mozilla-Status2. That may be the old way, as it appears to have a number of sqlite databases now.

Together with some of the other feature requests in #2087 (addressbook, filtering, etc.) this requires a separate, and perhaps more sophisticated, way to persist status and options than the simple flat config file we have now.

So some design decisions would have to be made first.
Low-priority, not easy, probably .36 at the earliest.

[zzz]

X-Mozilla-Status in the mail itself would be a lot of I/O, and we'd have to strip it on incoming mail so it isn't spoofed by spammers, as happened to t-bird years ago, see ​https://bugzilla.mozilla.org/show_bug.cgi?id=196749
So a separate file for per-mail status is preferable. Perhaps b64uidl:statusflags, one per line (take code from i2psnark.dht.dat) properties not a good idea as b64 contains =