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.
Currently at least the notmuch and maildir worker only set messages as read
if a body part is fetched. The gpg abstraction however fetches the full message.
We can simply set the readstate when we create the messageview, avoiding the issue.
Once this is merged, we can cleanup both workers.
