Only delete messages when unlabeled from trash/spam if they only exists
in All Mail and (spam or trash).
This patch also ports delete_from_trash.feature and use status rather
than fetch to count messages in a mailboxes.
When an IMAP client appends a message to a mailbox, it can specify
which flags it wants the appended message to have. We need to handle
these in a proton-specific way; not-seen messages need to be imported
with the Unread bool set to true, and flagged messages need to
additionally be imported with the Starred label.
