Force ordering when list widget column is sorted

When ordering is applied externally, ie. by a relation config, the orderBy call in the List widget simply adds an additional field to the ordering clauses, which prevents lists in these scenarios from being re-ordered correctly. This changes the order clause so that the ordering is reset and only the specified column is ordered when the user sorts a column.

Developers can continue to use the `extendQuery` event to do specialised custom ordering if required.

Fixes #4439.

modules/backend/widgets/Lists.php

@@ -532,6 +532,7 @@ class Lists extends WidgetBase
                 $sortColumn = $column->relation . '_count';
             }
 
+            $query->getQuery()->orders = [];
             $query->orderBy($sortColumn, $this->sortDirection);
         }