Fix small javadoc issue in ReferenceProperty

Fix: A small javadoc issue in the reference property class.
FeatureDefinition factory methods for properties

Add: Every property feature implementation (ObjectProperty, ReferenceProperty, TransientProperty) has two static createDefinition() methods for creating feature definitions without declaring an anonymous class just for that.
PropertyPersistenceTest uses property feature definition factories

Fix: The PropertyPersistenceTest uses the new feature definition factory methods that are supplied by the default property implementations.
Update changelog