Add Platform_qt, a baseclass for both Qt frontends

So we can share some code between QtQuick and QtWidgets

src/qtcommon/Platform_qt.cpp

@@ -0,0 +1,30 @@
+/*
+  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.
+*/
+
+#include "Platform_qt.h"
+
+#include <QGuiApplication>
+
+using namespace KDDockWidgets;
+
+
+Platform_qt::Platform_qt()
+{
+}
+
+Platform_qt::~Platform_qt()
+{
+}
+
+std::shared_ptr<ViewWrapper> Platform_qt::focusedView() const
+{
+    return qobjectAsView(qApp->focusObject());
+}