Add support for hidden objects which can be used as yaml anchors.
Inspired by the hidden items there should be a way to define objects which are not loaded. These can be used as yaml anchors and referenced by "proper" objects.
Imho and object name with a leading dot is invalid so this would be a good way to signal that this is an hidden object.
Hidden objects should work throughout the file.

Follow up for #4818 as requested by @jimtng