Improve FeaturePropertyAccessor for types/descriptors and @attributes

The idea is to make it possible to evaluate an expression holding an x-path against a complex feature type, and get a descriptor returned, in the same way that works for simple features.
However, polymorphism must be supported, i.e. when evaluating the x-path it should consider substitution groups.

The FeaturePropertyAccessor does not support getting @attributes from complex features, which is needed to use them in WMS style files.