Add NXQL ecm:parentPath

In NXQL we can deal with ancestors using either ecm:ancestorId if we have a document id available, or ecm:path STARTSWITH if we have a document path.

For just the immediate parent, we have ecm:parentId but there's nothing if we have a path only.

--> Add a ecm:parentPath pseudo-property, to be able to write ... WHERE ecm:parentPath = '/my/folder' and get as a result the documents in that folder.