Implement ScreenSaver fully

The proxy currently exposes the org freedesktop ScreenSaver interface, giving applications something to bind to, but then it blackholes the ActiveChanged events, making it even worse than if it didn't expose the object: applications will remain oblivious to the fact that it's not working, and won't try fallbacks.

This is needed for a correct operation of applications like Squeekboard, without having to code complicated capability testing and fallback paths.