Rewritten ETLayoutItem.valueTransformers serialization

* Added ETCollectionToPersistentCollection and ETItemValueTransformerToString

* Introduced ETValueTransformersToPersistentDictionary that leverages the two previous transformers for ETLayoutItem.valueTransformers persistency

Note: This fixes an exception in Collage. At launch time, COObject would fail to validate the type of the element in  'valueTransformers' against the metamodel (NSString vs ETItemValueTransformer).