This is an attempt at standardising the GIR XML in a way that we can
reliably validate; the schema is enough to validate simple GIR files,
but it still needs work for complex ones, like GTK's.

I've decided to use the compact Relax NG syntax as the schema format
because it's definitely easier to read and write that the DTD format.

Since the current XML version is 1.2, let's standardise on that instead
of making up a version 1.0.