flutter: Add initial WindowOverlayWidget.dart

Doesn't do much yet.

src/flutter/dart/lib/WindowOverlayWidget.dart

@@ -0,0 +1,30 @@
+/*
+  This file is part of KDDockWidgets.
+
+  SPDX-FileCopyrightText: 2019-2022 Klarälvdalens Datakonsult AB, a KDAB Group company <info@kdab.com>
+  Author: Sérgio Martins <sergio.martins@kdab.com>
+
+  SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only
+
+  Contact KDAB at <info@kdab.com> for commercial licensing options.
+*/
+
+import 'package:flutter/material.dart';
+import 'package:flutter/widgets.dart';
+
+/// @brief A Widget to host KDDW floating windows
+/// Since Flutter doesn't support real OS level multi-window we need
+/// to draw the floating windows into an overlay
+class WindowOverlayWidget extends StatefulWidget {
+  @override
+  State<StatefulWidget> createState() {
+    return WindowOverlayWidgetState();
+  }
+}
+
+class WindowOverlayWidgetState extends State<WindowOverlayWidget> {
+  @override
+  Widget build(BuildContext context) {
+    return Stack(children: []);
+  }
+}