TextBoxHelper with uppercase B

Renamed `TextboxHelper` to `TextBoxHelper` to match `TextBox`
I think a visual Studio extension added this

On 14:20, Sun, Nov 16, 2014 Thomas Freudenberg <notifications@github.com>
wrote:

> In MahApps.Metro.sln:
>
> > @@ -37,6 +37,9 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MahApps.Metro.Tests", "Maha
> >  	EndProjectSection
> >  EndProject
> >  Global
> > +	GlobalSection(CodealikeProperties) = postSolution
>
> This was added by Visual Studio 2013 automatically after I installed
> Update 4. Should I remove that section? I guess it will be re-added again
> as soon as someone else has Update 4 installed.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/MahApps/MahApps.Metro/pull/1650/files#r20409435>.
>