title: Polygon coordinates changed/edited/moved event (Google Maps V3)
body: <p>Since Google maps v3.11, it's possible to set the <code>draggable</code> property onto the <code>Polygon</code>, <code>Circle</code> and <code>Rectangle</code>. eg, <code>new google.maps.Polygon({ draggable: true })</code>.</p>
  
  <p>For <code>Circle</code> and <code>Rectangle</code>, there are events like <code>radius_changed</code>,<code>center_changed</code> and <code>bounds_changed</code> for us to subscribe when there is any change.</p>
  
  <p>But for <code>Polygon</code>, we can only subscribe the <code>set_at</code>,<code>insert_at</code> and <code>remove_at</code> events of <code>polygon.getPath()</code>.</p>
  
  <p>So, there will be a problem after I dragged the polygon, the position of the polygon had been changed/edited/moved, but no event to listen for this changes.</p>