libqtxdg is a Qt implementation of freedesktop.org XDG
specifications.

The library is able to use GTK+ icon theme caches for faster
icon lookup. The cache file can be generated with utility
gtk-update-icon-cache on a theme directory. If the cache is not
present, corrupted, or outdated, the normal slow lookup is still run.

libqtxdg (which supports qt5) can be installed alongside libqt6xdg
(which instead supports qt6).