Add property like isAdded to File and Transfer instead of checking identifier

[PimCoumans]

Instead of checking for the availability of the identifier property, a dynamic property like isAdded should assume this behavior. Perhaps this could be handled in a protocol that handles this for File and Transfer (which both have the optional identifier property)

[PimCoumans]

As mentioned by @AvdLee, the property identifier should be renamed to publicIdentifier and for a Board or Transfer object should have a isShared property in stead of isAdded