Savegame:Use saved diplstate order
Diplstate order is being saved to the savegame, but it's not used in the loading side.
Presumably the reason for that the saving side has been implemented in earlier versions already is for the benefit of newer versions loading such savegames - no need to keep the order present in "historical" versions listed in the newer version binary.
We should implement the loading side too. Targeting initially to master only, but should also confirm that S3_1 still has the same hardcoded order as the earlier versions (so it's ok to keep using that hardcoded order)