TextBoxHelper with uppercase B

Renamed `TextboxHelper` to `TextBoxHelper` to match `TextBox`
Ooops, sorry, you're right. I had installed [Codealike](https://codealike.com/) after reading [My Experience with Codealike](http://31og.com/post/my-experience-with-codealike). It helps me tracking how much time I spend on different projects. 

I'll remove it from this solution.