[YAML] Support hidden objects for yaml substitutions

[spacemanspiff2007]

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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