Update widgets.dart

Author: ashitaprasad

lib/widgets/widgets.dart

@@ -17,6 +17,7 @@ export 'dialog_about.dart';
 export 'dialog_history_retention.dart';
 export 'dialog_import.dart';
 export 'dialog_ok_cancel.dart';
+export 'dialog_proxy_settings.dart';
 export 'dialog_rename.dart';
 export 'dialog_text.dart';
 export 'drag_and_drop_area.dart';