This means Buddy and its subclasses no longer need to care about
NameOwnerChanged at all.
The old code might not have worked anyway, since it was watching for
NameOwnerChanged on the session bus, but invoking NM methods on the system bus.
This causes events in the log to be annotated with the module that emitted the
message.
Before: DEBUG - root: Starting up...
After: DEBUG - s-p-s.server_plugin: Starting up...
I've used a log domain of "sugar.presence..." for the client library and
e.g. "s-p-s.activity" for the service internals.
