With this change, NMConfig is really immutable and all
modifyable parts migrated to NMConfigData.
Another advantage is that components can now subscribe to
NMConfig changes to pickup changes to no-auto-default.
The NMConfig class should be immutable and its properties should
not change, with one exception: the NMConfigData property.
Later, when changing/reloading a configuration, NMConfig will only swap
the NMConfigData instance.
The NMConfigData instance itself is also immutable.