We now detect the component type of jMolecules' Association as association target type in AbstractPersistentProperty. AnnotationBasedPersistentProperty keeps the lookup of the explicit @Reference around but falls back to the Association analysis for that.
@Reference is now only considered by AnnotationBasedPersistentProperty. It was previously also (erroneously) considered by AbstractPersistentProperty.
It was also changed to return `null` in case no association target type could be looked up instead of returning the sole property's type in case that is an entity. This was done to satisfy the behavior documented on the interface but also to avoid the call to isEntity() during the calculation which might use association information in turn and thus lead to a stack overflow. Reworded the contract of PersistentProperty.getAssociationTargetType() to emphasize the symmetry with ….isAssociation() in implementation classes.
Closes: #2344.