[Notifications] Support job text for "items" (not just files)

And, in case of a single item/file, show full path (as a clickable hyperlink, too) if no destination is given.

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This way if there is a job with a single item but without destUrl (so no "Copying bar to Downloads") but has a URL in its details, it will show e.g. "Examining: Downloads/foo/bar" instead of "Examining: bar"

By supporting single KJob::Item, we can do special-cases for some KIO jobs, such as StatJob, which operate on a single file, and provide a more meaningful progress dialog for them.

Also, for jobs with multiple items (BatchRenameJob) it will show progress "Renaming: 5 of 123 items", with frameworks/kio!860 (merged)