title: TextField with animated hint / label
body: <p>I want to implement a form containing TextFields. Each field has a label / hint. I want the hint to animate and become a label when the user starts typing. This is a standard Material design pattern, so I expected it to be implemented by the standard Widgets.</p>
  
  <p>Something like this:
  <a href="https://i.stack.imgur.com/am51A.png" rel="noreferrer"><img src="https://i.stack.imgur.com/am51A.png" alt="enter image description here"></a></p>