flutter: Add WindowWidget.dart

A widget that represents a kddw window.
Specially since flutter doesn't support multi-window. We'll
have to draw them over an overlay.

src/flutter/dart/lib/WindowWidget.dart

@@ -0,0 +1,47 @@
+/*
+  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';
+import 'package:KDDockWidgets/FloatingWindow_flutter.dart' as KDDW;
+
+/// @brief A Widget that hosts a single KDDW FloatingWindow
+/// Since Flutter doesn't support real OS level multi-windows we need
+/// to draw the windows ourselves
+class WindowWidget extends StatefulWidget {
+  late final KDDW.FloatingWindow_flutter kddwFloatingWindowView;
+  WindowWidget(this.kddwFloatingWindowView);
+
+  @override
+  State<StatefulWidget> createState() {
+    return WindowWidgetState(kddwFloatingWindowView);
+  }
+}
+
+class WindowWidgetState extends State<WindowWidget> {
+  late final KDDW.FloatingWindow_flutter kddwFloatingWindowView;
+  WindowWidgetState(this.kddwFloatingWindowView);
+
+  @override
+  Widget build(BuildContext context) {
+    final x = kddwFloatingWindowView.x();
+    final y = kddwFloatingWindowView.y();
+    final width = kddwFloatingWindowView.width();
+    final height = kddwFloatingWindowView.height();
+
+    return Positioned(
+        left: x.toDouble(),
+        top: y.toDouble(),
+        width: width.toDouble(),
+        height: height.toDouble(),
+        child: kddwFloatingWindowView.flutterWidget);
+  }
+}