gdbusproxy: make g-name-owner property useful with unique names

Currently, GDBusProxy:g-name-owner only notifies changes to the unique name owner of the remote object in case the proxy was constructed for a well-known name. That sounds like an artificial restriction, and it's convenient to connect to notify::g-name-owner if a proxy instance has already been created for an unique name, instead of additionally using g_bus_watch_name() to track the owner.

To fix this, always connect to NameOwnerChanged after the proxy is initialized, instead of only doing so when the proxy was constructed for a well-known name.

https://bugzilla.gnome.org/show_bug.cgi?id=791316 #1310 (closed)