Add K_PLUGIN_CLASSES_WITH_JSON macro which generates a KPluginFactory and registers two plugins
Having two plugins is quite common for KCMs and having to type out the registrations manually is unneeded boilerplate.
Also, we do not need to type out the factory name. This is especially useful for static plugins, because there the factory name must match the plugin name for the K_IMPORT_PLUGIN statements to be correctly generated.