cache the XMLTokenMap data in static const vars, so we only need to
initialise them once

Change-Id: Ib63bedc5af5d5927fed189a6b1a86a5db6865c72
Reviewed-on: https://gerrit.libreoffice.org/71466
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>