@return $this for better IDE hinting

`@return self` confuses IDE to think that returned value is a BaseControl object. `@return $this` preserves the class and doesn't mark subsequent calls to getSeparatorControl() (and others) as invalid.

![After change](https://cloud.githubusercontent.com/assets/853656/5214559/4295e734-761e-11e4-8719-ba9402553eb2.png)
![Before change](https://cloud.githubusercontent.com/assets/853656/5214560/429864fa-761e-11e4-9e18-4e7c59290e3f.PNG)