(Small) Enhancement of sorting and filtering

I had to enable filter on a grid, it used resource which had relationship (and custom repository).  With the getPropertyMethod method it will possible to filter a grid with the custom alias (or not) :

Your grid :
``` html
...
<th>
{{ sylius_resource_sort('productName', 'product.field.name.label'|trans, null, null) }}
</th>
<th>
{{ sylius_resource_sort('c.name', 'product.field.name.label'|trans, null, null) }}
</th>
...
``` 
Your repository :
```php
public function getProduct() {
 $this->getCollectionQueryBuilder();
        ->innerJoin('p.category', 'c');
}
```