title: Qt (C++): Add background text in line edit
body: <p>I want to be able to add a line of (grey) text that when you type in the line edit, goes away. So the text is not actually there, it is just displayed. </p>
  
  <p>For example:<br>
  Before I type something, the grey background text is there:<br>
  <a href="http://i.imgur.com/dcXISZq.png" rel="noreferrer">http://i.imgur.com/dcXISZq.png</a></p>
  
  <p>After I type something, the text goes away, replaced by the text entered:<br>
  <a href="http://i.imgur.com/wfmvUsx.png" rel="noreferrer">http://i.imgur.com/wfmvUsx.png</a></p>