Added `yii\grid\DataColumn::items`

Allow  rendering label of values.

Example:
```php
/**
* @property integer $id
* @property string $username
* @property string $password
* @property integer $status
*/
class User extends ActiveRecord {

     const STATUS_INACTIVE = 0;
     const STATUS_ACTIVE = 1;  
  
}

// in view/user/index.php

<?= GridView::widget([
       'dataProvider' => $userProvider,
        'columns' => [
            'username',
            [
                'attribute' => 'status',
                'items' => [
                    User::STATUS_ACTIVE => 'Active', 
                    User::STATUS_INACTIVE => 'Inactive'
                ]                
            ],
        ]
]); ?>
```