title: Set value for ExtJS.form.ComboBox
body: <p>I have an Ext.grid.EditorGriPanel which has an combobox editor in its columnModel (the store of combox has just only 2 items: enabled/disabled). When a new blank row is inserted to grid, I want the combobox in that column have to be set to a default value (enabled), and the dropdown is not expanded.
  I've tried to use
   <code>grid.colModel.columns[2].editor.select(0)</code> 
  to get the combobox editor and set "enabled" value for it, but it did not work. Another way is using <code>grid.colModel.columns[2].getCellEditor(rowIndex)</code>, but it returns an EditorGrid (I was so surprised because the ExtJS 3.3.1 API doesn't have this component).
  Could you please help me on this problem?
  Thank you so much!</p>