Port MainWindowMDI to the new architecture

Has a qtquick implementation now.

src/qtquick/views/MainWindowMDI_qtquick.h

@@ -0,0 +1,39 @@
+/*
+  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.
+*/
+
+#ifndef KD_MAINWINDOW_MDI_QTQUICK_H
+#define KD_MAINWINDOW_MDI_QTQUICK_H
+#pragma once
+
+#include "views/MainWindowMDI.h"
+#include "qtquick/views/MainWindow_qtquick.h"
+
+namespace KDDockWidgets {
+
+
+/// @brief MainWindow sub-class which uses MDI as a layout
+class DOCKS_EXPORT MainWindowMDI_qtquick : public Views::MainWindow_qtquick, public Views::MainWindowMDI
+{
+    Q_OBJECT
+public:
+    using Views::MainWindowMDI::addDockWidget;
+
+    ///@brief Constructor. See base class documentation
+    explicit MainWindowMDI_qtquick(const QString &uniqueName, QQuickItem *parent = nullptr,
+                                   Qt::WindowFlags flags = Qt::WindowFlags());
+
+    ///@brief Destructor
+    ~MainWindowMDI_qtquick() override;
+};
+
+}
+
+#endif