As some issues indicate (#6195#10471#11008#6861) there's an issue with
the autoincrement save slot feature: slot index will increase and very old
saves won't be deleted.
This commit adds support to delete old save states with a user defined
save state limit (global). Instead of wrapping around the slot counter
it will simply delete the oldest save, since it is simpler.
For now there's a limit of one deletion per save, which ensures a user
cannot delete many saves by accident if they set the limit too low.
1c5099051c