similar to FontFeaturesDialog.
FWIW gtk's viewport implementation automatically overwrites the step
size to be 10% of the visible height (and page size of 90% of the
visible height)
Change-Id: If356724a47b0a10069f129bed4a40cfdee57bfa8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/148138
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>