Allow to hide First, Previous, Next and Last buttons.

Hi, now there is no way to disable text buttons in CLinkPager (without development own pager).

My propose:

```php
$this->widget(
    'zii.widgets.CListView',
    array(
        'dataProvider' => $dataProvider,
        ...
        'pager' => array(
            'firstPageLabel' => false,
            'lastPageLabel' => false,
            'header' => '',
            'footer' => '',
            'cssFile' => false
        )
...
```

In this case First and Last links will be disabled.

Cheers.

close #2630