graph/formats/gexf12: new package for handling GEXF1.2 graph description files

Created by: kortschak

This was largely written by aqwari.net/xml/cmd/xsdgen working on the XSD files available from the gephy/gexf specs repository, with some help; the specs use includes which cause xsdgen to fail, so I manually in-lined the includes.

The generated code was then edited to make optional attributes exist on pointers, to reduce the length of labels to within reasonably normal Go idiom and removing unnecessary namespacing.

The package is versioned since the format appears to arbitrarily be updated.

Please take a look.