Plasma/libplasma2/dataengineimplicitlyshared

Discussion

The challenge will be in DataEngine subclasses which may have class members that are not copyable, let alone the bloat of the copying or the challenge of keeping that data in each copy in sync with the shared data in DataEnginePrivate. So to do this properly, we may need to break out DataEngine which consumers use from DataEngine that gets subclassed.