Alleviate the annoyance of Job Finished notifications

Whenever a job disappears for whatever reason (cancelled, finished, error, ...) we display
a persistent "Job Finished" notification. When the user explicitly cancels the job, there's
no need to confirm since the job will be gone. This solution is not ideal since we just track
it inside the plasmoid (ie. when you click Cancel in a "Override this folder?" dialog, it will
still show the notification) but at least the obvious "You don't say" moment is gone that way.

Ideally in the future whoever is responsible for the job (KIO, ...) should be given the
responsibility for an appropriate notification since the owner knows best why a job quit.