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Some documentation tool (as hotdoc[0]) need to have information about
symbol declaration and documentation positions in the source files
to be able to do smart indexing (automatically build the documenation
index).

[0] https://hotdoc.github.io/

Fixes #175

This updates the schema to validate all GIRs currently
available in Debian unstable except a few obviously broken ones.

For testing the following tools were used:
  $ trang gir-1.2.rnc gir-1.2.rng
  $ xmllint --noout --relaxng gir-1.2.rng Gtk-3.0.gir

https://bugzilla.gnome.org/show_bug.cgi?id=766313

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.