Suggest people to use K_PLUGIN_CLASS_WITH_JSON

Summary:
It's simpler to understand than K_PLUGIN_FACTORY_WITH_JSON that asks
you for a factory name but you never know which name to use, until you
realize the name doesn't matter at all.

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18076