Added datagrid_column_type_action_cell_action_widget to twig

``datagrid_column_type_action_cell_action_widget`` is somethig that allows theming single actions in ``action`` column type. 

Lets asume we have action ``edit``. 
Up to now to create such actions the following configuration of action column type was required: 

```
$datagrid->addColumn('actions', 'action',array(
    'label' => "Actions",
    'field_mapping' => array('id'),
    'actions' => array(
        'edit' => array(
            'url_attr' => array(
                'class' => 'btn btn-warning btn-small-horizontal',
                'title' => 'crud.list.datagrid.action.edit'
            ),
            'content' => '<span class="icon-eject icon-white"></span>',
            'route_name' => 'fsi_admin_crud_edit',
            'parameters_field_mapping' => array(
                'id' => 'id'
            ),
        )
    )
));
```

From now it also can be done by: 

```
$datagrid->addColumn('actions', 'action',array(
    'label' => "Actions",
    'field_mapping' => array('id'),
    'actions' => array(
        'edit' => array(
            'route_name' => 'fsi_admin_crud_edit',
            'parameters_field_mapping' => array(
                'id' => 'id'
            ),
        )
    )
));
```

with following block in DataGrid theme template file: 

```
{# Resources/views/datagrid.html.twig #}

{% block datagrid_column_type_action_cell_action_edit %}
    {% spaceless %}
        <a href="{{ attr.href }}" title="{{ 'crud.list.datagrid.action.edit'|trans }}" class="btn btn-warning btn-small-horizontal">
            <span class="icon-eject icon-white"></span>
        </a>
    {% endspaceless %}
{% endblock %}
```

``url_attr`` and ``content`` options are still available in action configuration.