Introduce K_PLUGIN_CLASS_WITH_JSON

Summary:
Does the same as K_PLUGIN_FACTORY_WITH_JSON but saves you typing the class name twice.

If you look at lxr.kde.org you can see our codebase is full of stuff like

K_PLUGIN_FACTORY_WITH_JSON(NinjaBuilderFactory, "kdevninja.json", registerPlugin<NinjaBuilder>(); )

where we repeat NinjaBuilder twice, so add a nice macro so that we only have to write it once

Reviewers: dfaure, mlaurent

Reviewed By: dfaure

Subscribers: mlaurent, kossebau, apol, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10333