Use progress, no key dialog, as primary UI for reencrypt job

The progress dialog must stay around for the entire lifetime of the job
(see status display updates in reencrypt()). The code had already been
(wrongly) assuming this to be the case, before.

Also, if a job is still at the stage of showing the key dialog, it has
not even started working yet. Start over fresh to avoid ambigouities in
that case (user may have changed folder, meanwhile).