title: Extjs: only show checkbox when another field has a value
body: <p>i have a grid with a checkboxcolumn, all works fine but i would like to only show the checkbox if another field has a certain value. I work with version 3.3.1 but i guess that an example from another version would get me started.
  If not possible, disabling the checkbox would also be fine.
  Do i have to do that in a renderer or a listener and how ?</p>
  
  <pre><code>var checkColumn = new Ext.grid.CheckColumn({ 
   header: 'Checklist OK ?',  
   dataIndex: 'checklist_ok',  
   width: 20, 
   align: 'center' 
  }); 
  
  cmDiverse = new Ext.grid.ColumnModel({ 
   defaults: {"sortable": true, "menuDisabled":false, "align":"right"}, 
   store: storeDiverse, 
   columns: [ 
     {"id":"id", "header": "id", "hidden": true, "dataIndex": "id", "width": 20}, 
     checkColumn, 
     ... 
  
  gridDiverse = new Ext.ux.grid.livegrid.EditorGridPanel({ 
    id              : "gridDiverse", 
    enableDragDrop  : false, 
    loadMask        : true, 
    clicksToEdit    : 1, 
    layout          :'anchor', 
    cm              : cmDiverse, 
    .... 
  </code></pre>