Forked from CMIS-759

Define a comment as having the following characteristics:
­- a comment is a dependent object. It has a single parent object and should be automatically deleted by the repository when that parent is deleted.
­- a comment can have a parent comment (to support multiple comment threads on a given document).
­- a comment has a single string/text property containing the comment text.
­- if the repository imposes a maximum length for comments, the text property metadata will indicate it

• comments do not contain content streams