Make the XML interfaces the single source of truth

The XML descriptions of DBus interfaces and the actual C code that implements them have gone out of sync in the past.

Some tasks to fix this:

• Revive the scripts to generate introspection.[ch] from the XML files.
• Maybe remove hardcoded version="0.1.7" attribute from introspection XML (see below).
• Don't remove annotations from XML for introspection (see below).
• Replace the copies of introspection.[ch] in the various modules with code generated at build time.
• Audit C code for each interface/method to ensure it matches the XML; increase test coverage to do this. Offloaded to #75
• Replace hand-written DBus code with GDBus code generation. Offloaded to #76

I'll start working on this.