[Notifications] Provide basic job information in tooltip

Especially for longer downloads where a user is more likely to minimize the job popup, it is useful to be able to see basic job information (title, i.e. "Downloads", percentage) without having to open the notification history.

Moreover, it's hard to judge completion percentage from the tiny circle.

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• Job title is only shown if it is the same for all jobs (e.g. multiple downloads or multiple file copies). Otherwise a generic "n running jobs" is shown as before
• Percentage is averaged between all running jobs