Edit order_group_article result from orders screen

Following #253, this improves the "sort by ordergroups" and "sort by articles" views, used in orders and balancing and allows in-place editing of the result (when the order is finished but not closed).

The image here is about balancing, but the same applies to viewing an order.

![orders_edit_groups](https://f.cloud.github.com/assets/503804/1986446/24ebc498-844c-11e3-9136-3a1cc5f556bd.png)

Would this fix #29 as well?