Report the time a job has been running

KJob::elapsedTime() returns zero unless a job calls startElapsedTimer() from its start() implementation. Kup never called it, so the job tracker sent an elapsed time of zero to the notification on every progress update.

Plasma works out the average speed and the time remaining from that field, so a running backup showed an average speed of 0 B/s, and the speed was written out twice, once in the chart and once under Details, because the notification takes a zero there for a job that reports no speed over time.

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Noticed in https://blogs.kde.org/2026/09/12/this-week-in-plasma-6.8-beta-release/ that kup job were lacking some data.