This makes it easier to read, because we do not have to handle a list of
files that get added to the targets. In other KDE projects, we also
prefer the target centric approach.
In case one wants to reuse the same category in multiple places, it
would be best to create a small static lib. But for now, this is not
needed.
The moc process already rebuilds the plugin when the JSON file changes, consequently the additional parameter is not needed.
Task: https://phabricator.kde.org/T14649
