title: QML property - read-only, assignment, binding
body: <p>If I had a rectangle with a property width, there are three options to set a value that I confuse:</p>
  
  <pre><code>read-only property int widthReadOnly: 200
  
  Rectangle{
      width: 200                             //first 
      width: widthReadOnly                   //second
      Component.onCompleted: {width = 200}   //third
  }
  </code></pre>
  
  <p>Could you tell me when to use each of them? 
  Thank you.</p>