Change shift-drop .zip to clear instead of append

Appending by default makes more sense now that artpacks loads the most
recently appended pack first, so the new pack will be visible
immediately -- but any existing artwork from the original pack will
still be fallen back to, if necessary. With artpacks-ui and
voxel-artpacks the pack ordering is now revealed and can be altered, so
having this extra state (vs the previous default of clearing all loaded
packs) is now more reasonable.