Minor change to how we show you sales stats in the design screen

I’ve changed the calculation and display of this number:

It now shows you the sales in the previous hour, for the first 10 hours of the game, after that it shows you the average over the last 10 hours.

This makes it (i think) much more useful in cases where you are batching cars from a large number of designs, and you may only be selling 1 of that design this hour, but 10 the next hour, and so on.