We probably don't want the KIO Widgets job tracker as fallback as
that won't be any more useful than what the browser has buit-in.

Furthermore, set immediate progress reporting, so that the job is shown
right away and always, i.e. even when downloading a small file will show
a "Finished" notification with useful "Open" action.

Finally, while at it get rid of "id" in `DownloadJob` as it was never used.