Fire backend.list.extendRecords event during export useList (#4148)

Provides an opportunity to modify and / or return the $results collection object before the controller exports it. Credit to @fansaien

modules/backend/behaviors/ImportExportController.php

@@ -629,6 +629,11 @@ class ImportExportController extends ControllerBehavior
 
         $query = $widget->prepareQuery();
         $results = $query->get();
+        
+        if ($event = $widget->fireSystemEvent('backend.list.extendRecords', [&$results])) {
+            $results = $event;
+        }
+        
         foreach ($results as $result) {
             $record = [];
             foreach ($columns as $column) {