[DataGrid widget] Add getGrid method

When you create a `DataGrid` widget, there is currently no way to communicate with the `Grid` it uses. This allows you to do stuff like:

```php
$datagrid = $this->makeWidget('Backend\FormWidgets\DataGrid', $config);
$datagrid->getGrid()->bindEvent('grid.dataChanged', function($changes) {
    ...
});
```