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.

As it turns out, the only fields of these messages that the UI cares
about (right now) is the name.

A sequence-set is an IMAP-specific implementation detail. Throughout the
UI, aerc simply operates using lists of opaque identifiers. In order to
loosen the coupling between the UI and IMAP in particular, replace most
usages of imap.SeqSet with []uint32, leaving the translation to a SeqSet
to the IMAP backend as needed.

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.

Assuming we always have a sorted dirlist
(other code depends on that already), we don't need to loop over the
dirStore.
Any filtering done should be performed elsewhere

Also sets the public List() method to return the unfiltered
list of directories directly from the store.

Credit for this fix goes to Reto; I guess if we're not gonna be mutt
we should probabaly do things correctly.