Introduce twip/mm100 conversion functions instead of duplicated macros

Change-Id: Ib689e35b417e0e9016cd6a239c986e0603a99d62
Reviewed-on: https://gerrit.libreoffice.org/8837
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>