LayerFeatureConstraints not parsed for NamedLayer

Seems like implementation was written and used for UserLayer at some point but NamedLayer is not using it... though I guess it should?

Attaching patch enabling NamedLayers parsing to not throw UnsupportedOperationException and instead parse the LayerFeatureConstraints tag using existing implementation.

Testcase already exists, all that needs to be done for it to be caught is to add a LayerFeatureConstraints to the namedLayers.sld test-data file - attaching that too.