[KStandardItemListWidget] Request the pixmap size we want and let the icon loader scale it

I noticed that depending on the configured icon size it would spend a significant amount of time in KPixmapModifier::scale.
I don't see a point in requesting a fixed icon size and then scale it down manually as opposed to having the KIconLoader do the scaling for us.
Especially for SVGs it could then even serve us a properly rendered SVG for this size rather than a scaled down pixmap version.

Differential Revision: https://phabricator.kde.org/D22116