Specify scripted effect kcm using X-KDE-ConfigModule
At the moment, if an effect wants to have a configure button in the desktop effects kcm, it needs to add X-KDE-PluginKeyword field with its plugin id. This is confusing.
This change ports scripted effects to X-KDE-ConfigModule, which is more intuitive than X-KDE-PluginKeyword.
kcm_kwin4_genericscriptedconfig is a special config module that loads config.ui from KPackage. However, an effect can specify a different config module in the metadata if kcm_kwin4_genericscriptedconfig doesn't suit their needs.