Add log entries for quantity changes and product removals

It appears that when an admin changes the quantity of an item under the order "edit" tab and then saves their changes, this is not reflected in the log at all. I thought at first that I was mistaken, but upon inspection of uc_order_edit_products(), it doesn't seem to do any logging before calling uc_order_product_save() to save the updated product info.

I'm classifying this as a bug, because everything else in the system is logged (adding products, changing address info, changing tax, etc), so this seems like a pretty big omission to me.