Revisiting parentItem

[rrlevering]

This is an extension of #851 because that issue seems to be closed/old.

The current state that left off was "Comment parentItem Comment". This still doesn't make full sense because "CreativeWork comment Comment" is essentially the inverse edge of this (I don't know if that was the intention but it's very close semantically). If the top level item that the comment is attached to is not a Question/Answer (let's say a DiscussionForumPosting or a VideoObject or something), there's no way to specify it.

I am proposing expanding the range of parentItem to be CreativeWork.

[danbri]

This makes sense. I also added :Question and :Answer explicitly, despite it being implied by their being kinds of Comment, so as to match the text "The parent of a question, answer or item in general.".

[Seirdy]

@danbri Ideally, parentItem would be applicable to a wider variety of types than Answer, Comment, and Question. A reply to something could be an email, an article, a meme, etc. It could also replace itemReviewed for reviews.