Rework the way effect configs are loaded

Currently we load an effect's config by going over all effect config plugins and picking one with the right X-KDE-ParentComponent.

This uses KPluginTrader, a class we want to get rid of for KF6. It also seems overly complex. It seems much more natural to me to have the effect specify its config instead of the other way around.

This patchset adds new metadata for config modules to the builtin effects, loads the config module metadata from plugin effects using a new X-KDE-ConfigModule key while keeping compatibility for plugins that are not yet ported