title: How to show the Keyboard automatically for a Textfield in Flutter
body: <p>I have a <code>TextField</code> in Flutter of which I want to automatically select the text and show the keyboard. 
  I can select the text through a <code>TextEditingController</code>, but even with a <code>FocusNodes</code> requestFocus the keyboard isn't shown, when the Widget opens. </p>
  
  <p>How to automatically open the keyboard for a TextField?</p>