Added id and class option to collection items

In order to be able to have tabable collections there is the need for a collection-item id and the need for classes
to be added to a collection.

If you want to have tabs you can get to this with the following javascript code.

```javascript
$(document).ready(function () {
    updateCollectionTabs('init');
});

$('#vendor_corebundle_formtype_collection_control_group').on('add.mopa-collection-item', function(e, row) {
    updateCollectionTabs('add', row[0]);
});

$('#vendor_corebundle_formtype_collection_control_group').on('remove.mopa-collection-item', function(e, row) {
    updateCollectionTabs('remove', row[0]);
});

function updateCollectionTabs(type, row)
{
    $('#CollectionTabs').empty();
    $('#vendor_corebundle_formtype_collection_control_group .collection-item').each(function(index, value) {
        $('#CollectionTabs').append('<li><a href="#' + value.id + '" data-toggle="tab"> #' + (index + 1) + '</a></li>');
    });

    switch(type) {
        case 'add':
            $('#CollectionTabs a[href="#' + row.id + '"]').tab('show');
            break;
        case 'remove':
        default:
            $('#CollectionTabs a:first').tab('show');
            break;
    }
}
```

In addition to that you need to set the following options in the form builder:

```php
            $builder->add(
                'collection',
                'collection',
                array(
                    'widget_controls_attr' => array(
                        'class' => 'tab-content',
                    ),
                    'widget_items_attr' => array(
                        'class' => 'tab-pane',
                    )
                )
            )
```

And last but not least an empty ul container in the template:

```html
            <ul id="CollectionTabs" class="nav nav-tabs"></ul>
```

Based on the preparation you can adjust this to your needs.