Preliminary dbus-releated cleanups
I've been doing some exploratory refactoring of the dbus-related code in atspi, with this purpose:
- Move things in the direction of extracting the dbus marshaling/demarshaling code, similar to what I started in registryd.
- Remove a bit of unused code, superfluous checks, etc.
- Correlate the above with the code coverage report to see where there are missing tests, or definitely unused code paths.