exclude stock orders from order sum in invoices and financial balances

[mjavurek]

I like the feature of stock orders and we often use it in our FC. During balancing, the stock orders should be excluded from the total amount of the order, because no existing ordergroup will be charged for it: When we add both order and stock delivery to the invoice (example in foodsoft demo), the stock order is considered twice: once in the order and a second time by the stock delivery. As a result, a loss for the FC appears with the amount of the stock order.

Under unpaid invoices, the balance seems to be prefect because no difference is displayed:

This is because the stock order is included and the delivery is excluded from the balance. Instead, the stock order should be excluded, but the delivery should be included. In this example this leads to the same result, but if there is no stock order for the delivery or if stock order and delivery differ, the balance is incorrect.

[mjavurek]

Implementation: #1075