kcms/icons: find larger icons and fall back to svg if there is none

The icon KCM needs to filter out small icons in previews because in
some themes there are only small pixmaps and scalable icons, and
MatchBest will return paths of small pixmaps, which doesn't look good
in the preview grid.

Ref: https://gitlab.gnome.org/GNOME/adwaita-icon-theme/-/commit/74aa69851dd37b863eb071fae4c2365f5bc04030


(cherry picked from commit c10830cb)