libnm-util, libnm-glib: Make NMRemoteConnection instantiable by NMObject

In order to resolve NMRemoteConnection-valued properties, NMObject
needs to be able to create NMRemoteConnections. But NMObject assumes
that all the objects it will be creating have "dbus-connection" and
"dbus-path" properties. So add those properties to NMRemoteConnection,
aliasing the existing "bus" and "path" properties (and ensure that
whichever version gets set, we keep that value, rather than letting it
get overwritten by the NULL default value of the other one).

https://bugzilla.gnome.org/show_bug.cgi?id=693669

libnm-util/nm-connection.c

@@ -1658,7 +1658,8 @@ set_property (GObject *object, guint prop_id,
 
 	switch (prop_id) {
 	case PROP_PATH:
-		nm_connection_set_path (connection, g_value_get_string (value));
+		if (g_value_get_string (value))
+			nm_connection_set_path (connection, g_value_get_string (value));
 		break;
 	default:
 		G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);