PropertyValue: Specify if value is default or not

[ljacqu]

PropertyValue wraps a value returned by Property and specifies whether it was "fully valid" in the resource, but a false cannot be distinguished whether it comes from the resource (with errors) or if the default value had to be used.

For migrations, it's important to know whether there is a value (fully valid or not) in the resource. "Fully valid" in that case makes it misleading and it is not possible to answer this question besides extending BaseProperty to make getFromReader public, which is ugly.

To be revised together with #480