writerfilter: convert fasttokens to Python

It's shorter and f9bf15e19ec823a58ee32bf94da81f3bb1a147bc (writerfilter:
initial strict DOCX support, 2014-03-07) shows it's a pain to do anything
non-trivial with XSLT 1.0 -- in that case black magic was needed to do a simple
unique sort.

Change-Id: Icf4e7b580ce1db6826989500dbf4a012d79acdb9