If a MailboxInfo has the attribute \Noselect,
it is not possible to use this name as a selectable mailbox.

Therefore it should not be passed to the directory handlers.
The issue pops up if one has a hierarchy like this:
INBOX
INBOX/lists/stuff
INBOX/lists/otherStuff

Even though lists is not a valid inbox (doesn't contain mail, only other maildirs)
it will show up in the directory listing, when we iterate over the MailboxInfo.

It does have the corresponding attribute set though and we can simply filter it out.

I was partway done implementing a UI for users to approve untrusted
certs with, but let's just make them configure their servers right
instead.

Worker.callbacks contains two types of callbacks: some are action callbacks,
some are message callbacks. Each of those is access from one side of the
communication channel (UI goroutine vs. worker goroutine).

Instead of using a channel, we can use two different maps for each kind. This
simplifies the code and also ensures we don't call an action callback instead
of a message callback (or the other way around).

Message IDs are allocated for both messages from UI to workers and the other
way around. Hence, the global nextId variable is accessed from multiple
goroutines.

Instead, use atomic to access the global counter.