Recalculation of dish ingredients through number of servings doesn't cascade to purchase lists

[MarkusLeupold]

To reproduce this bug, follow these steps:

1. Create a dish with at least one ingredient which we call i
2. Create a purchase list and add i to it
3. Recalculate the dish to a different amount of servings than before

Although the amount of i has changed in the dish, the purchase list doesn't reflect this change. The change is actually completely invisible inside the purchase list feature of Coocook: Even the "unassigned items" section doesn't contain the missing part of i. The only way to get the right value onto the purchase seems to be, to first remove i from the list and to re-add it again after that.

Therefore it impossible to prepare purchase lists before the final numbers of servings are known, which makes this bug exremely severe.