Put ApplicationScreenshots inside a ScrollView
This lets it use normal scrolling behavior instead of the unusual and awkward behavior of ListView's internal Flickable, and also lets us get rid of the custom ScrollBar and a superfluous ColumnLayout.
Found this while investigating https://bugs.kde.org/show_bug.cgi?id=439195, which seems to be caused by the
LayoutMirroring.enabled property not propagating correcting inside ApplicationPage. Haven't figured out why yet.