Add queryable cmis:isPrivateWorkingCopy property

When doing a query, the repository MAY include private working copies in the result when PWCSearchable=true (2.1.9.9). However for such a repository, there's no way to filter specifically for the presence or absence of PWCs. We miss something like ... WHERE cmis:isPrivateWorkingCopy = true.

This would also be useful when there are many private working copies available to a user, because getCheckedOutDocs cannot be used for complex queries.

Finally, this is also useful when you have just a Document and wish to know if it's a PWC or not (it's easier than comparing cmis:versionSeriesCheckedOutId and cmis:objectId).

Proposal: add cmis:isPrivateWorkingCopy to the domain model. Suggest that repositories should make it queryable.