Add scenario settings to set custom boat and/or horse images

[Celtic Minstrel]

Essentially, in the scenario details window, you could set the number of the picture used for the boats and horses. The picture format would be the same as for the built-in horse and boat graphic, and probably numbering would be something like 1000+x for a custom graphic.

That would mean you can make the boat look like a horse if you really want to, or the horse look like a boat. It also means there's a way to use the new ship graphic.