MerchantGui onTrade only called once on shift click

I don't know if this was intended, but currently the onTrade method of MerchantGui is only called once if someone uses shift + click to complete a trade more than once, which could lead to trades being miscounted if a dependent mod was counting on that method for counting trades (sorry if there was a different intended way to do that, there didn't seem like an obvious one to me)