title: qml: Can not access styleData in slider componet
body: <p>I need the value of slider's handle width, but even if I just copy the example code from Qt document, the debugger still tell me:</p>
  
  <blockquote>
    <p>Cannot read property 'handleWidth' of null</p>
  </blockquote>
  
  <p>What did I do  wrong?</p>
  
  <p>My code as below   </p>
  
  <pre><code>import QtQuick 2.0
  import QtQuick.Controls.Styles 1.4
  import QtQuick.Controls 1.4
  
  Slider {
      anchors.centerIn: parent
  
      style: SliderStyle {
          groove: Rectangle {
              implicitWidth: 200
              implicitHeight: 8
              color: "gray"
              radius: 8
          }
          handle: Rectangle {
              anchors.centerIn: parent
              color: control.pressed ? "white" : "lightgray"
              border.color: "gray"
              border.width: 2
              implicitWidth: 34
              implicitHeight: 34
              radius: 12
  
              Text{
                  text:"test"
                  anchors.right:parent.right
                  anchors.rightMargin:  styleData.handleWidth * 0.3
              }
          }
      }
  }
  </code></pre>
  
  <p>UPDATE:
  I found a workaround in the end.
  Use state and propertychange will allow us to change the item's properties under the "handle" property from slider level</p>