[th/connection-path-as-ref-str] libnm: use NMRefString for nm_connection_get_path()

NMConnection is an interface, implemented by NMSimpleConnection and NMRemoteConnection.

For the most part, an NMConnection is only the content of the profile (the settings). The "path" of the connection refers to the D-Bus path, and wouldn't really make sense of the NMConnection interface or the NMSimpleConnection type.

As such, the daemon (which only uses NMConnection and NMSimpleConnection) never sets the path. Only libnm does.

NMClient uses NMRefString extensively for the D-Bus interface and the path is already internalized. Take advantage of that. It is very likely, that we are able to share the path instance in libnm at which point it makes sense to use NMRefString.

Also, during nm_simple_connection_new_clone(), we can just take another reference instead of cloning the string.