Add GML format in GetFeatureInfo requests

To fix http://hub.qgis.org/issues/7719

As we can see in the issue it's a bit confusing that the feature are partially formated in the `qgswmsserver.cpp` for the XML part, and in the `qgshttprequesthandler.cpp` for the txt/html part.

I see that's needed for the `qgssoaprequesthandler.cpp`, actually I don't have any problem with this but if we want to have a geojson output it will be difficult to do it properly.

In my first commit I copy things from the `qgswfsserver.cpp` but finally I change some things on it and the important part in in `qgsogcutils` than I think that ok like it.

I don't know if you have some coding style than it will probably have an issue with it.