Added defaultbindingproperty

Added [DefaultBindingProperty("Text")] to MetroTextBox to allow it to be chosen at the Data UI customization of a datasource: http://msdn.microsoft.com/query/dev12.query?appId=Dev12IDEF1&l=EN-US&k=k(vs.datasource.howtoaddcustomcontrol);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.5);k(DevLang-csharp)&rd=true