[Icons KCM] Load preview pixmaps for animation on-demand and cache them

No need to load 18 preview pixmaps of which 12 can only be seen during the animation.
Furthermore, cache the pixmaps as ListView deletes and recreates delegates as you scroll,
leading to us recreating the icon pixmaps as you scroll up and down with many
themes installed

Differential Revision: https://phabricator.kde.org/D13793