containments/folder: Use KNotificationJobUiDelegate on drop

Gives us a Plasma notification when dropping a file onto the desktop fails rather than a modal message box.

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• Dropped a file onto a broken link, got a Plasma notification error instead
• Dropped a file that already existed, got an “overwrite?” dialog as before

The job delegate API seems to still be centered around QWidget :/ so it seems I cannot parent the dialog to the desktop window…