add removing hook "preRemove" & "preBatchRemove" for ListAction

I just got the same problem with [#issue 1314](https://github.com/sonata-project/SonataAdminBundle/issues/1314), and I added the removing hook.

You can put a function "preBatchRemove" in your Admin class, and it will be involved with a preameter `$idx` while you are deleting something in ListAction. 
```php
public function preBatchRemove($idx){
    foreach($idx as $id){
        // do sth ...
    }
}
```

<br />
if no function "preBatchRemove" created,  the "preRemove" will be involved as usual, maybe it's lack of performance, but anyway, it just works fine.
```php
public function preRemove($object){
   // ...
}
```