so we don't need to cast at the call sites.

Also fix the type of mpExtInputRing in SwDoc

Change-Id: I60794a4b5c24ca552eafa87b8810dc08efdee3ea
Reviewed-on: https://gerrit.libreoffice.org/52455Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>