[Notifications] Hide job info label for successive jobs

When multiple jobs of the same kind are listed, visually group them.
While this doesn't actively group them (i.e. having a copy, a move, and a copy again
won't move the second copy job to the first - there's no real model underneath, so we
cannot just drop a sort proxy model ontop),
it still improves the most likely situation of copying multiple different things at once.

Differential Revision: https://phabricator.kde.org/D6109