More mail flags can now be set, unset, and toggled, not just the
read/seen flag.
This functionality is implemented with a new `:flag` and `:unflag`
command, which are extensions to the matching `:read` and `:unread`
commands, adding support for different flags. In fact, the
`read`/`unread` commands are now recognized aliases to `flag`/`unflag`.
The new commands are also well documented in aerc(1).
The change mostly extends the previous read/unread setting functionality
by adding a selection for the flag to change.
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.
