More per-map tileart fixes

What this merge request does is revert the backing up and restoring of waloff, walock and picsiz for per-map tile loading, and improve the per-map art backups such that only tile slots which were actually overridden for per-map art are restored. All other tiles are left alone. This fixes both the issue described in 298d80a2, as well as #66 (closed).

Also includes a minor change regarding previousboardfilename.