title: pyqt5 / qtdesigner textbox default label
body: <p>I am trying to create a GUI for my python program. One of the tools that I need is a text input box. </p>
  
  <p>Now, I want a text label for this box saying "Please insert texts." Is there a function to add a label that shows inside the input textbox as default and disappear when user click the box to type?</p>
  
  <p>I don't mind to use qt designer or pyqt5 coding. </p>
  
  <p>Thank you guys.</p>