Support child nodes of a document variant to NOT be copied from preview to draft and vice versa

To avoid that when a draft is created from a preview the potential hst:page gets overridden to the draft and on save from the draft to the preview, we need to support to suppress nodes from being copied between preview and draft. Between unpublished and published these nodes must be copied.

When can support this by introducing a mixin, something like

hippo:skipdraft