title: QML ItemDelegate highlighted property not work
body: <p>I want to customize highlight color in <code>ItemDelegate</code>. If I use default <code>ItemDelegate</code> with Material theme then all ok and color change when i hover over this item, but when I redefine background it breaks down and colors not change anymore.</p>
  
  <p><em>MyItemDelegate.qml</em>:</p>
  
  <pre><code>import QtQuick 2.11
  import QtQuick.Controls.Material 2.4
  import QtQuick.Controls 2.4
  import QtQuick.Templates 2.4 as T
  
  T.ItemDelegate {
      id: myItemDelegate
      height: 40     
      anchors.left: parent.left
      anchors.right: parent.right
  
      contentItem: Text {
              text: "Hello"
              anchors.fill: parent
              verticalAlignment: Text.AlignVCenter
              horizontalAlignment: Text.AlignHCenter
          }
      }
  
      background: Rectangle {
          anchors.fill: myItemDelegate
          color: myItemDelegate.highlighted ? "blue" : "transparent"
      }
  }
  </code></pre>
  
  <p>Why <code>highlighted</code> property not work? And how I can customize this color?</p>