Allow arbitrary properties in referential constraints

Referential constraints are currently limited to primitive properties, potentially nested within complex properties, which requires e.g. to "resolve" referential constraints between complex properties or single-valued navigation properties into one <ReferentialConstraint> element per primitive property of the complex property, and key property of the related entity type.

An obvious generalization is to also allow complex properties, meaning that each member property has the same value, and as a consequence single-valued navigation properties, meaning that the dependent navigation property has the same target as the principal navigation property. This is just a short-hand notation for adding referential constraints on all leaf structural / key properties.