title: Ext JS: Adding listener to a Picker component in order to disable a separate button
body: <p>I want to add a listener to my School component which is a column inside a grid. School's <strong>editor</strong> field's xtype extends Ext JS Picker. Basically, the listener will have a function that sets <strong>bindBool</strong> to true when a classroom value is bound. </p>
  
  <p>The grid:</p>
  
  <pre><code> xtype: 'grid',
              scrollable: 'y',
              maxHeight: 300,
              margin: 10,
              sortable: false,
              plugins: [{
                  ptype: 'cellediting',
                  clicksToEdit: 1,
                  pluginId: 'gridEditor'
  </code></pre>
  
  <p>The editor </p>
  
  <pre><code>{
    header: 'School',
    width: 300,
    dataIndex: 'classroom',
    renderer: function(value) {
        return value.classroomString;
    },
    editor: {
        xtype: 'classroomfield',
        bind: {
            value: '{classroom}'
            }
        }
  }
  </code></pre>
  
  <p>The Zulu button:</p>
  
  <pre><code>items: [{
      xtype: 'panel',
      buttons: [{
          style: 'margin-right: 30px',
          text: 'Zulu',
          iconCls: 'x-fa fa-upload',
          handler: 'xray',
          bind: {
                  disabled: '{bindBool}'
          }
      }]
  }]
  </code></pre>