[library] Use Xapian for library (fulltext) search in title/description

[kelson42]

It is currently based on regexp expressions which is quite inefficient. As a replacement the Kiwix-lib should maintain a in-memory Xapian index, updated each time books are loaded/removed from the library. I think this should be done during the OPDS backend implementation.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.