How to override XPM Cancel Button?

Question

I would like to do something when the user clicks "Cancel Editing" button in Tridion 2013 XPM. Below is my configuratin file and JS file. The issue is JS not fired when i click the "Cancel Editing" button.

Congiguration File

    <?xml version="1.0"?>
    <Configuration                 xmlns="http://www.sdltridion.com/2009/GUI/Configuration/Merge"  xmlns:cfg="http://www.sdltridion.com/2009/GUI/Configuration" xmlns:ext="http://www.sdltridion.com/2009/GUI/extensions" xmlns:cmenu="http://www.sdltridion.com/2009/GUI/extensions/ContextMenu">
<resources cache="true">
<cfg:filters/>
<cfg:groups>
    <cfg:group name="xpmextension.CommandSet">
        <cfg:fileset>
            <cfg:file type="script">/js/xpmextension.js</cfg:file>
            <cfg:file type="reference">xpmextension.Interface</cfg:file>
        </cfg:fileset>
        <cfg:dependencies>
          <cfg:dependency>Tridion.Web.UI.Editors.CME</cfg:dependency>
          <cfg:dependency>Tridion.Web.UI.Editors.CME.commands</cfg:dependency>
        </cfg:dependencies>
    </cfg:group>  
</cfg:groups>   
   </resources>
 <definitionfiles/>
  <extensions>
   <ext:editorextensions>
  <ext:editorextension target="SiteEdit">
    <ext:editurls />
    <ext:listdefinitions/>
    <ext:taskbars/>
    <ext:commands/>     
    <ext:commandextensions>
    <ext:commands>
      <ext:command name="CancelEditing" extendingcommand="xpmextension"/>
    </ext:commands>
    <ext:dependencies>
      <cfg:dependency>xpmextension.CommandSet</cfg:dependency>
    </ext:dependencies>
    </ext:commandextensions>        
    <ext:contextmenus/>
    <ext:lists/>
    <ext:tabpages/>
    <ext:toolbars/>     
    <ext:ribbontoolbars/>           
  </ext:editorextension>
</ext:editorextensions>
<ext:dataextenders/>
 </extensions>  
 <commands>
<cfg:commandset id="xpmextension.intereface">
<cfg:command name="xpmextension" implementation="Extensions.xpmextension"/>
<cfg:dependencies>
<cfg:dependency>xpmextension</cfg:dependency>
</cfg:dependencies>
</cfg:commandset>
 </commands>
 <contextmenus/>
 <localization/>
 <settings>
<defaultpage>/Views/Default.aspx</defaultpage>
<navigatorurl>/Views/Default.aspx</navigatorurl>
<editurls/>
<listdefinitions/>
<itemicons/>
<theme>
  <path>css</path>
</theme>
<customconfiguration/>
</settings>
</Configuration>

Below is my JS file

Type.registerNamespace("Extensions");

  Extensions.xpmextension = function Extensions$xpmextension()
  {
   Type.enableInterface(this, "Extensions.xpmextension");
   this.addInterface("Tridion.Cme.Command", ["xpmextension"]);
  };
  
  Extensions.xpmextension.prototype.isAvailable = function xpmextension$isAvailable(selection) {
  return true;
  }
  
  Extensions.xpmextension.prototype.isEnabled = function xpmextension$isEnabled(selection) {
  return true;
  }

      Extensions.xpmextension.prototype._execute = function xpmextension$_execute(selection) 
      {      
      alert("You have clicked the button");
   alert(window.location.href);
      
      }