Fix #2048 (streak bonuses not appearing to add up)

Addresses the issue of misleading streak notifications, by making the "gp" notification style take two arguments: value and bonus. (It's thus important to pass a 0 if we're not dealing with a streak. If there's a better way to handle that, I'm all for it!) The GP notification subtracts off the bonus before displaying, resulting in an alert that contains only the pre-streak total. When we then add the "+X Streak Bonus!" notification, the total list of alerts correctly adds up to the amount of GP gained, where previously the streak bonus was getting double-listed.

If you watch very closely, totaling up every alert you get, there will be discrepancies of a few Silver due to rounding. But you'll be much closer than you previously would, where you'd seem to be short several Gold!