[TASK] Fill UpdateStockReaction with functionality

[rintisch]

In the branch 5.x-dev an UpdateStockReaction was introduced. At the moment this is more of a boilerplate without real functionality.

It aims to add a reaction which can be used to update the stock from a 3-rd party application. The base is now already in place, the real functionality needs to be implemented.

[extcode]

I'll add the necessary code.