Takes load time from 3m to 2m11

(*) Add a list sorted by which/start
(*) Use it in lcl_GetTextAttrs
(*) Fix GetLastPosSortedByEnd, previously we could potentially
static_cast the search item to something it is not
(*) Add assert to SwpHints::DeleteAtPos to verify that we don't need
sorting of the main list when deleting.

Change-Id: If82ea650172ca88486507f31b45cac8d22682451
Reviewed-on: https://gerrit.libreoffice.org/73152
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>