title: Checkbox selection model - check column header text
body: <p>I have a checkbox selection model that I am using on a gridpanel. I have configured the CheckboxModel like so: </p>
  
  <pre><code>Ext.create('Ext.selection.CheckboxModel', {
      mode: 'Single',
      checkOnly: true,
      showHeaderCheckbox: false
  });
  </code></pre>
  
  <p>This configuration allows the user to select only one record from the gridpanel. I have removed the checkbox in the header since I want the user to have to explicitly check the box in the record row. Now I have just a blank header in that column....is there any way to add text to that header column? From what I can tell it seems like my only options are to either leave the checkbox in the header or remove it and have a blank column header...is that correct?</p>