Use a ListView for the screenshots gallery
This has the advantage of simplifying the code considerably. This also unify the style with apps.kde.org and make the screenshot use the full height of the containers, so they don't loook tiny anymore We are losing the shadow affect on the screnshot, but considering that the screenshots could have already shadow, this remove the double shadow effect. This also adds some navigation button on the desktop because horizontal scrolling is not always an intuitive navigation pattern on a desktop.
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