c366c155f1
PROP_0 is how we commonly name this property when we don't use
NM_GOBJECT_PROPERTIES_DEFINE(). Rename it.
Also, allow to skip PROP_0 in nm_gobject_notify_together(), that
is handy to optionally invoke a notification, like
nm_gobject_notify_together (obj,
PROP_SOMETHING,
changed ? PROP_OTHER : PROP_0);