editor: prevent any registration of objects on the system bus

D-Bus access-control is name-based; so requests for a specific name
are allowed/denied based on the rules in /etc/dbus-1/system.d.  But
apparently apps still get a non-named service on the bus, and if we
register *any* object even though we don't have a named service,
dbus and dbus-glib will happily proxy signals.  Since the connection
editor shouldn't ever expose anything having to do with connections
on any bus, make sure that's the case.