title: Issue with extjs checkbox header
body: <p>I'm currently implementing part of a project in <code>ExtJS</code>, the requires me to modify an existing grid to only show checkboxes on rows with a certain status(different than AN, NP, RS), this status is determined by the value in record.data.codiceStato : </p>
  
  <pre><code>selectionModel = Ext.create('Ext.selection.CheckboxModel', {
      checkOnly: true,
      listeners: {
          select: function (sm, idx, rec) {
              alert("Look ma!");
          },
          beforeSelect: function (sm, idx, rec) {
              if (idx.data.codiceStato == 'RS' || idx.data.codiceStato == 'AN' || idx.data.codiceStato == 'NP')
                  return false;
          }
      },
      renderer: function (value, metaData, record, rowIndex, colIndex, store, view) {
          if (record.data.codiceStato != 'RS' &amp;&amp; record.data.codiceStato != 'AN' &amp;&amp; record.data.codiceStato != 'NP')
              return '&lt;div style="margin-left: -1px;" class="' + Ext.baseCSSPrefix + 'grid-row-checker"&gt; &lt;/div&gt;';
  
          else
              return '';
      }
  });
  </code></pre>
  
  <p>I have written a check in my <strong><code>beforeSelect</code></strong> listener in order to avoid the checkbox selecting the rows that do not have this status and it works. The only problem is with the header checkbox now, in fact, when I click on it the first time it enables all the rows with a checkbox, but then it doesn't uncheck them when I click on it again. Any solutions ? Thank you </p>