OPAL-2082: CriterionDropdown widget and CategoricalCriterionDropdown widget

...get
![image](https://f.cloud.github.com/assets/2565497/1553243/b0b5e514-4e30-11e3-9dd5-0d2fe19a9f53.png)

![image](https://f.cloud.github.com/assets/2565497/1553248/bf672a96-4e30-11e3-896f-4dce473f586e.png)

![image](https://f.cloud.github.com/assets/2565497/1553260/d5c77ec6-4e30-11e3-9ba8-1e800f5bd7d1.png)