Impress slide-sorter page-down/up

[mmeeks]

Please do check for correct behavior vs. the LibreOffice slide sorter, but what I expect when that is focused is that page-down/up moves to the next/previous slide - and not some big chunk of slides down/up (as might normally happen in a scrolled area)

Thanks :-)

[mmeeks]

As we discussed it at COOL-days :-)