Resolve "Veranstaltungsexport für Admins berücksichtigt nicht AdminCourseWidgetPlugins"

Closes #3531

Merge request studip/studip!2419

app/controllers/admin/courses.php

@@ -880,7 +880,9 @@ class Admin_CoursesController extends AuthenticatedController
         $filter_config = Request::getArray('fields');
 
         if (count($filter_config) > 0) {
-            $courses = AdminCourseFilter::get()->getCourses();
+            $filter = AdminCourseFilter::get();
+            PluginEngine::sendMessage(AdminCourseWidgetPlugin::class, 'applyFilters', $filter);
+            $courses = $filter->getCourses();
 
             $view_filters = $this->getViewFilters();