React to the special-collections discovery job and updating the sorting.

This sorts IMAP trash folders correctly in the folder list.
(Not that this matters much to me, but it clarifies the obvious "race"
in the code). I checked all other callers of folderIsTrash, and they
happen on user action (selecting folders, deleting messages), so there's
nothing to update. The user just has to wait for the kmail startup to be
finished.