title: qml progress in the ProgressBar is jump
body: <p>I use the <code>ProgressBar</code> as <code>QtQuick.Controls 1.4</code>.<br>
  In qml:</p>
  
  <pre><code>ProgressBar {
      id: myProgressBar
      Layout.fillWidth: true
      minimumValue: 0
      maximumValue: 100
      visible:true
  }
  </code></pre>
  
  <p><br>
  And there a c++ class:<br></p>
  
  <pre><code>class MyProgress: public QObject
  {
      Q_OBJECT
      Q_PROPERTY(int progress READ GetProgress NOTIFY progressChanged)
  public:
      explicit MyProgress(QObject *parent = 0);
  
  public:
      int GetProgress();
  signals:
      void progressChanged(const int progess);
  private:
      int m_progress;
  
  };
  </code></pre>
  
  <p><br>
  In <code>MyProgress</code>, have a <code>QThread</code> to update the 'progress'.<br>
  Then ,in qml ,I add a event handler:<br> </p>
  
  <pre><code>MyProgress {
      id: progress
      onProgressChanged:{
          myProgressBar.value = progress
      }
  }
  </code></pre>
  
  <p><br>
  It works.But,the progress in the <code>ProgressBar</code> looks like jump,jump,jump instead of linear increasement.<br>
  I think that bacause the progress is too fast that the GUI thread has no time to refresh.<br>
  Does anyone has some advice?</p>