Rebase FieldDetails upon an abstract and generic version of itself.

This should be useful for cases where the value has a very specific type
(eg, for PostalAddressDetails.postal_addresses). We'll create subclasses of
FieldDetails for those cases.

When we've created all the appropriate derived classes, we'll remove
FieldDetails entirely.

Helps: bgo#653679 - Change PostalAddressDetails.postal_addresses to
support vCard-like arbitrary parameters