I add a new class method deserialize_create() to GimpConfigInterface which
returns the GimpConfig object per deserialization, instead of modifying an
existing bare object.

This matters for cases like our GimpResource (and later our GimpItem) classes
which are fully managed by libgimp and should be unique objects per actual
resource. It should even be possible to compare the pointer itself for identity.
That's why we need to let GimpResource create the object (in reality request it
to the infra and only ref it) through this new class method.

With this commit and the previous ones, all GimpResource are now properly stored
as plug-in settings (e.g. the "film" plug-in has a font setting which is now
properly remembered).

These identifiers are not portable (across various installations and therefore
not for XCF either), but at least they are reasonably identifying data on a same
installation (unlike GimpResource's int ID which is only valid within a single
session) which makes them very fine for plug-in settings storage.

When a data file disappears, we fallback to the context default data instead.