Edit order_group_article result in 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.

- [x] update totals after change in "sort by" views
- [x] allow adding articles - Q: what user-interface approach to use? (see below image)
   - **A: postpone**
- [x] Q: should editing of group_order_article results be restricted to balancing?
   - **A: no, require finance permission**
- [x] Q: do we need a new role for updating group_order_article result? !=finance !=orders
   - **A: no, only allow updating with finance permission**

![orders_edit_groups](https://f.cloud.github.com/assets/503804/1986542/53bb9dd8-844d-11e3-9731-39b6a8ec749b.png)

Would this fix #29 as well?

I'm uncertain how to ui design adding an article. The select2 dropdown listbox is used at different places, and I like it. But here it would mean that each and every ordergroup or article would get one - which is a bit too much. This could be fixed by showing the edit boxes only after pressing an "edit" button, which would also add an "add article" selectbox, but I think that is less clear.

Any ideas on this? @JuliusR @robwa @dokterbob?