Fix theme selector theme lookup

knewstuff puts downloaded assets into subdirs like
`.local/share/kblocks/themes/brlublur-hotstuff-access/` which we found
correctly using a subdir iterator, but then failed to preserve the subdir
portion for later loading through KGameTheme in the same function.
to make sure KGameTheme can properly resolve the full path we need to pass
the desktop file path *relative* to data dir we found it in.

REVIEW: 129238