flutter: Use a mixin for the View's data

Group_flutter.dart doesn't inherit from View_flutter.dart, it inherits
directly from C++, so wasn't getting the data.

Since Dart doesn't support multiple inheritance we'll use a mixin

src/flutter/dart/lib/View_mixin.dart

@@ -0,0 +1,88 @@
+/*
+  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:KDDockWidgets/PositionedWidget.dart';
+import 'package:KDDockWidgetsBindings/Bindings.dart' as KDDockWidgetBindings;
+
+class View_mixin {
+  late final PositionedWidget flutterWidget;
+
+  Function(int, int)? updatePositionCallback;
+  Function(int, int)? updateSizeCallback;
+
+  int m_x = 100;
+  int m_y = 100;
+  int m_width = 400;
+  int m_height = 400;
+  bool m_fillsParent = false;
+
+  int m_maxHeight = 16777215;
+  int m_maxWidth = 16777215;
+
+  setSize_2(int width, int height) {
+    print("View_mixin::setSize called ${width}x${height}");
+    if (m_width != width || m_height != height) {
+      m_width = width;
+      m_height = height;
+      if (updateSizeCallback != null) updateSizeCallback!(width, height);
+    }
+  }
+
+  void setWidth(int width) {
+    setSize_2(width, m_height);
+  }
+
+  setHeight(int height) {
+    setSize_2(m_width, height);
+  }
+
+  void setFixedWidth(int width) {
+    // TODO
+    setSize_2(width, m_height);
+  }
+
+  setFixedHeight(int height) {
+    // TODO
+    setSize_2(m_width, height);
+  }
+
+  setGeometry(KDDockWidgetBindings.QRect geo) {
+    final sz = geo.size();
+    final topLeft = geo.topLeft();
+    setSize_2(sz.width(), sz.height());
+    move_2(topLeft.x(), topLeft.y());
+  }
+
+  KDDockWidgetBindings.QRect geometry() {
+    return KDDockWidgetBindings.QRect.ctor4(m_x, m_y, m_width, m_height);
+  }
+
+  KDDockWidgetBindings.QRect normalGeometry() {
+    // TODO
+    return geometry();
+  }
+
+  KDDockWidgetBindings.QSize maxSizeHint() {
+    print("View_flutter::maximumSize()");
+    return KDDockWidgetBindings.QSize.ctor2(m_maxWidth, m_maxHeight);
+  }
+
+  move_2(int x, int y) {
+    print("View_flutter::move called ${x},${y}");
+    if (m_x != x || m_y != y) {
+      m_x = x;
+      m_y = y;
+      print(updatePositionCallback);
+      print(updateSizeCallback);
+      if (updatePositionCallback != null) updatePositionCallback!(x, y);
+    }
+  }
+}