FeatureId support in GML (r/w) and KML (w)

Now that we have FeatureId support (see #733), we can add this back to the
GML parsers (v2 and v3). Also add write support for FeatureId in KML, read
support was already added by @fredj
Merge pull request #737 from bartvde/feature-id

FeatureId support in GML (r/w) and KML (w) (r=@fredj)