ExtJS 4.1 gridpanel如何在每列最后添加带文字的操作按钮

Ext.create('Ext.grid.Panel', {
    title: 'Action Column Demo',
    store: Ext.data.StoreManager.lookup('employeeStore'),
    columns: [
        {text: 'First Name',  dataIndex:'firstname'},
        {text: 'Last Name',  dataIndex:'lastname'},
        {
            xtype:'actioncolumn',//这里就是放置按钮的地方
            width:50,
            items: [{
                icon: 'extjs/examples/shared/icons/fam/cog_edit.png',  // Use a URL in the icon config
                tooltip: 'Edit',
                handler: function(grid, rowIndex, colIndex) {
                    var rec = grid.getStore().getAt(rowIndex);
                    alert("Edit " + rec.get('firstname'));
                }
            },{
                icon: 'extjs/examples/restful/images/delete.png',
                tooltip: 'Delete',
                handler: function(grid, rowIndex, colIndex) {
                    var rec = grid.getStore().getAt(rowIndex);
                    alert("Terminate " + rec.get('firstname'));
                }
            }]
        }
    ],
    width: 250,
    renderTo: Ext.getBody()
});