implemented comprehensive accessibility (A11y) support across the app

lib/view/pages/foreign_timetable/element_picker_page.dart

@@ -166,6 +166,7 @@ class _ElementPickerPageState extends State<ElementPickerPage> {
                     ? null
                     : IconButton(
                         icon: const Icon(Icons.clear),
+                        tooltip: 'Leeren',
                         onPressed: () {
                           _searchController.clear();
                           setState(() => _query = '');