Takes load time from 1m45 to 22s

Instead of (1) taking a list of SwTextAttrs (2) invoking a method that
inserts a new one (3) taking a new list and doing a delta on the list,
just the information about the new SwTextAttr up the stack and avoid all
that work.

Change-Id: Iec88618af696299aaba65f43bc33b1a5f030fac0
Reviewed-on: https://gerrit.libreoffice.org/73195
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>