The structure of a GAPI XML file is now defined by the XML schema in
gapi.xsd.
This XSD is now used by the generator to do a first sanity check on an
XML file before trying to generate code from it. Each generatable object
still does its own validation.
The XSD can also be seen as a documentation of the GAPI XML format, and
can be used by third-parties that produce GAPI XML to validate their
output.
