Fixed #258 - Added logic to try and prevent overloading the sprite cache when randomizing minions in enemy-abundant chapters. (So far, only FE8's Ch. 20 is affected)

As the logic is fairly restrictive in the class selection, I've only applied this to one chapter in all of the GBAFE games, as it's the only chapter that exhibits this issue (at least the only one reported to have this issue). The theory is that, due to the large number enemies on the screen, randomizing minion classes randomly will likely increase the types of sprites on the map. This will overload the sprite cache and cause some graphics to become corrupted randomly. It's mostly an aesthetic issue, as the underlying data is should be fine. As far as I know, only FE8 exhibits this issue, and only if there are an abundance of unique sprites needed for a chapter. The logic simply locks the number of different classes the normal enemies can be assigned if the option is enabled. The vanilla game has 9 different regular enemy types, so that's where the limit came from.