- Add maildir flags to complement a messages imap flags
- Set the "seen" flag on sent messages when using the maildir backend
- Cleanup AppendMessage interface to use models.Flag for both IMAP and
maildir
A sequence-set is an IMAP-specific implementation detail. Throughout the
UI, aerc simply operates using lists of opaque identifiers. In order to
loosen the coupling between the UI and IMAP in particular, replace most
usages of imap.SeqSet with []uint32, leaving the translation to a SeqSet
to the IMAP backend as needed.