title: Disable docked button in a view from controller
body: <p>I have a MVC app built with extJS 4.
  I want to disable a docked button in a grid panel from a controller
  Here is my grid panel view:</p>
  
  <pre><code>Ext.define('SDI.view.MissionsGridPanel', {
  extend: 'Ext.grid.Panel',
  alias: 'widget.missionsGridPanel',
  width: 688,
  title: 'Missions',
  store: 'MissionsStore',
  tbar:[
                  {
                      text:'Delete mission',,
                      icon: '/images/delete.png',
                      itemId: 'removeMissionButton',
                      disabled: true,
                      action :'delete'
                  }
              ]...
  </code></pre>
  
  <p>Here is my controller :</p>
  
  <pre><code>Ext.define('SDI.controller.MissionsController', {
  extend: 'Ext.app.Controller',
  views: ['SDI.view.MissionsGridPanel'],
  refs: [
      {
              selector:'missionsGridPanel',
              ref:'missionsGridPanel'
        },
      {
              selector:'missionsGridPanel button[action=delete]',
              ref:'missionsGridPanelToolbarDelButton'
        }
      ],
  init: function() {
  
       this.control({
          'missionsGridPanel': {
              selectionchange: this.onMissionSelect
          }
  
  })
  },
  
  onMissionSelect: function(pTarget,pRecord,pOptions){
      console.log("Mission is selected")
      this.getMissionsGridPanelToolbarDelButton().setDisabled(false);
  }
  
  });
  </code></pre>
  
  <p>I don't understand why this doesn't work.
  "Mission is selected" is logged but the button remains disabled.</p>