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.