Add Threefish encryption

ODF 1.2 Part 3 section 4.8.1 "manifest:algorithm-name" lists identifiers for the various supported encryption algorithms.

We should add an identifier for the Threefish cipher. This algorithm, from the same author as Blowfish, also comes with a copyright-free implementation and the author has declared that he has not patented it:

https://www.schneier.com/threefish.html

Scheier also recommends Threefish over the Blowfish.

For backwards compatibility we probably will still want to default to Blowfish in cases where the algorithm-name attribute is missing.