Add a Directory Listing object to the Zim Archive metadata for Xapian indices

[Jaifroid]

This is a sub-issue of #769. In order to detect whether we need to load libzim for Xapian search, we need to be aware of the existence of a Xapian listing. We currently have titlePtrPos and articlePtrPos as attributes of the ZIM file object for the Directory Listings that they point to. We should add an appropriate attribute pointing to the Xapian data, which could be simply the pathname, that would only be present if the loaded ZIM archive has a Xapian index, and would be null if there is no Xapian index.