Edit citation dialog: improve item order for usability sake

[michael]

Here it would be good to order by selected in the first place, label in second place and alphabetically when there's no label.

Currently you have to scroll around to find the selected items for a citation. Now if we apply the sorting all the time we would have the problem that the interface jumps. I think what we should do is only refreshing the order when the dialog is opened the next time.