Fire extendQuery hook after select()

I'm trying to call `$query->addSelect()` inside the Lists widget's `extendQuery` event however you currently call `$query->select($selects);` [after extendQuery](https://github.com/octobercms/october/blob/master/modules/backend/widgets/Lists.php#L421) - removing my added select fields. `extendQuery` always allow for complete customisation of the query by running last.