allow selected operator for form action conditions

Works in all contexts of form action conditions (both in normal modules and advanced modules):
**Opening a case**

![image](https://cloud.githubusercontent.com/assets/249606/3257429/7acd7468-f22b-11e3-8e29-6bd9fc4d0c71.png)

**Closing a case**

![image](https://cloud.githubusercontent.com/assets/249606/3257437/927875ea-f22b-11e3-80f4-b44152c4782a.png)

Results in xpath like `selected(/data/question4, 'item6')`