The operation() method on the MailBackend trait returns a trait object
that can read or modify an Envelope directly from the backend. This is
used to get eg the envelope's text, or set flags. It has two arguments,
envelope hash and folder hash.
Only the Maildir backend needed the latter argument, and it can be replaced with a dictionary to match envelope hashes to folder hashes within the Maildir backend.
Add a boolean field to accounts that states if the account can be
accessed. Local backends (Maildir/mbox) return true every time, but
remote backends (IMAP) may not. Accounts start as offline and then get
initialised when their status goes to online. Right now if an IMAP
account startup but later get offline, there are crashes. With this
change the account can be switched back to offline when that happens.
Split the TlsStream of the live IMAP connection to an enum to allow both
offline and online states. The connection is restarted if offline when
requested.
Path of folder was compared to folder names and saving to eg
'INBOX/Sent' would not result into mail being saved to Folder { name:
'Sent', path: 'INBOX/Sent' }
If an email is removed by an outside actor, it is marked as removed.
Renaming files first send a Remove event and then a Rename one. So if a
removed email turns out to have been renamed by someone else, issue a
Create event to get it back.
Remove events almost always come immediately before Rename events,
showing that the previous name of a file is removed and then renamed.
Keep proper tabs by marking removed paths instead of actually removing them.
