Serde supports automatically converting types after/before (de-)serialization to a proxy type.

This way, we could more easily support Serde in cxx-qt-lib, after our previous failed attempts in #425 .
It would be somewhat less optimal, as e.g. serializing a QString would mean a conversion to/from String, but still better than not supporting it.

@Montel does this sound like a feature you'd be interested in adding?