[core] generic annotations for Labels and Relations

[CrystalMethod]

We previously talked about this during a private conversation, but I would like to put it up for public discussion:

I would like to propose generic annotations for Labels (or better: Roles) and Relations. Aggregation (or better: composition) of domain objects is one of the key features of XO and it should be used in the same way across the different datastore implementations.

[CrystalMethod]

Querydsl Annotation Processing would be much easier with just two Annotations for Label (Entity, Role) and Relationship.

[CrystalMethod]

The "Property" annotation is another candidate for a generic annotation. Pretty much all datastore implementation will provide some kind of renaming.