Adds the possibility to set a specific limitation for all choices

![screenshot_groupchoice](https://f.cloud.github.com/assets/4263772/495295/3196feb8-bbbd-11e2-9a63-e856a69aff8e.png)

This solution works with javascript. After setting a limit, you can change it for every option. Our users wanted this option because they often work with many (>20) groups but the same limit for every choice.

This solution works but has some difficulties:
-Works only with javascript
-I am not that trained on javascript and I have to admit that this piece of code is more like quick & dirty (in this case not really quick. It took quite some time to get to work with YUI)
-The given limit is not being checked for a numeric input (it is checked when you finaly try to create the group choice with given groups and limits)
-I use a new language string (at the moment just in english)