Port MainWindowMDI to the new architecture

Has a qtquick implementation now.

src/qtwidgets/views/MainWindowMDI_qtwidgets.h

@@ -0,0 +1,49 @@
+/*
+  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.
+*/
+
+/**
+ * @file
+ * @brief MainWindow sub-class which uses MDI as a layout
+ *
+ * @author Sérgio Martins \<sergio.martins@kdab.com\>
+ */
+
+#ifndef KD_MAINWINDOW_MDI_QTWIDGETS_H
+#define KD_MAINWINDOW_MDI_QTWIDGETS_H
+#pragma once
+
+#include "views/MainWindowMDI.h"
+#include "qtwidgets/views/MainWindow_qtwidgets.h"
+
+namespace KDDockWidgets {
+
+namespace Views {
+
+/// @brief MainWindow sub-class which uses MDI as a layout
+class DOCKS_EXPORT MainWindowMDI_qtwidgets : public Views::MainWindow_qtwidgets, public Views::MainWindowMDI
+{
+    Q_OBJECT
+public:
+    using Views::MainWindowMDI::addDockWidget;
+
+    ///@brief Constructor. See base class documentation
+    explicit MainWindowMDI_qtwidgets(const QString &uniqueName, QWidget *parent = nullptr,
+                                     Qt::WindowFlags flags = Qt::WindowFlags());
+
+    ///@brief Destructor
+    ~MainWindowMDI_qtwidgets() override;
+};
+
+}
+
+}
+
+#endif