Currently the dirlist ignores the counts provided by the dirInfo.
However some of the workers can actually provide accurate counts much quicker
than if we count the flags.

Eventually we will also want to enable displaying counts for background folders,
where the brute force counting won't work as none of the headers are fetched yet.

This commit models it in an opt-in manner, if the flag isn't set then we still
count the messages manually.

Exposes the notmuch tags accordingly, stubs it for the maildir worker.

Before, we were using several IMAP-specific concepts to represent
information being displayed in the UI. Factor these structures out of
the IMAP package to make it easier for other backends to provide the
required information.

Before, the information needed to display different parts of the UI was
tightly coupled to the specific messages being sent back and forth to
the backend worker. Separating out a models package allows us to be more
specific about exactly what a backend is able to and required to
provide for the UI.