Por all usages of QWindop/windowHandle to KDDockWidgets::Window

Removes a lot of asQWidget() code and is also prepared for non-Qt
views.

src/qtquick/Window_qtquick.cpp

@@ -0,0 +1,33 @@
+/*
+  This file is part of KDDockWidgets.
+
+  SPDX-FileCopyrightText: 2020-2022 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.
+*/
+
+#include "Window_qtquick.h"
+
+#include <QWindow>
+
+using namespace KDDockWidgets;
+
+Window_qtquick::~Window_qtquick()
+{
+}
+
+std::shared_ptr<ViewWrapper> Window_qtquick::rootView() const
+{
+    return {};
+}
+
+Window::Ptr Window_qtquick::transientParent() const
+{
+    if (QWindow *w = m_window->transientParent())
+        return Window::Ptr(new Window_qtquick(w));
+
+    return nullptr;
+}