title: redraw function in canvas api
body: <p>I am trying to make a timer in qml using the canvas API(the same one which is used in html5).I need to redraw the screen every second or so.Is there any function which could update the screen with newly fed parameters?
  For example I am using the arc function where I specify the angle for the clock arc to be drawn:</p>
  
  <pre><code>ctx.arc(150, 150, 95, 0,1.57,false);
  </code></pre>
  
  <p>In this,the angle will change every second or so.</p>