Dynamic snippets: only they are put to payload

```
{snippet container}
    {foreach [1, 2, 3] as $id}
        {snippet dynamic-$id} ... {/snippet}
    {/foreach}
{/snippet}
```

Now, you have to invalidate only static snippet `container`. With my alteration, you have to invalidate both static snippet and dynamic snippet - and only dynamic snippet is put to payload. I believe it's the ussual desired behavior. Also, when you invalidate only static snippet, it adds all dynamic snippets as one item to payload (simply whole content of static snippet).

Example:

```php
$this->invalidateControl('container');
$this->invalidateControl('dynamic-2');
```

results to `payload`:

```js
{
    snippets: {
        'snippet--dynamic-2': ...
    }
}
```