Cascade form groups on FormMapper::remove call

Currently, when a form field is removed, the corresponding form group field is not removed. Moreover, when a form group has not more fields, it is not removed as well. This PR fixes this, and consequently fixes empty form tabs :

![capture](https://f.cloud.github.com/assets/663607/1782004/bdbe9b4e-68a1-11e3-9fe0-6690b94e7451.PNG)
![capture](https://f.cloud.github.com/assets/663607/1782008/e5161cd0-68a1-11e3-8177-41b46d51a96f.PNG)