Validate our GIR files
The GIR schema is incomplete, and we have no idea if the GIR files we generate in our own repository are even correct.
Let's fix this:
- finish the GIR schema, so that it covers all the GIR files we have in our repository, as well as the ones we generate at build time
- validate every GIR file we touch
Edited by Emmanuele Bassi