added frontend

hartmann-foto-documentation-frontend/lib/utils/global_stack.dart

@@ -0,0 +1,16 @@
+class GlobalStack<T> {
+  final _list = <T>[];
+
+  void push(T value) => _list.add(value);
+
+  T pop() => _list.removeLast();
+
+  T peek() => _list.last;
+
+  bool get isEmpty => _list.isEmpty;
+
+  bool get isNotEmpty => _list.isNotEmpty;
+
+  @override
+  String toString() => _list.toString();
+}