title: identify if QVariant is String or Int
body: <p>I will do a method that write data on modbus.
  I need that QVariant return if is String or Int.</p>
  
  <p>My var.:
  <strong>QVariant dataToWrite</strong></p>
  
  <p>Exist a method that return dataToWrite content is String or Integer?
  In c# I did how this solution: <a href="https://stackoverflow.com/questions/894263/how-do-i-identify-if-a-string-is-a-number">How do I identify if a string is a number?</a></p>
  
  <p>I saw documentation on this link: <a href="http://doc.qt.io/qt-5/qvariant.html" rel="noreferrer">http://doc.qt.io/qt-5/qvariant.html</a>
  But in qt i didn't find solutions.</p>
  
  <p>Thanks</p>