Add Platform and Platform_qtwidgets

So we can start removing QtWidgets vs QtQuick ifdefs.
Each "platform" will implement the differences

src/qtwidgets/Platform_qtwidgets.cpp

@@ -0,0 +1,36 @@
+/*
+  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 "Platform_qtwidgets.h"
+#include "KDDockWidgets.h"
+
+static KDDockWidgets::Platform_qtwidgets s_platformQtWidgets;
+
+using namespace KDDockWidgets;
+
+Platform_qtwidgets::Platform_qtwidgets()
+{
+#ifdef DOCKS_DEVELOPER_MODE
+    if (qEnvironmentVariableIntValue("KDDOCKWIDGETS_SHOW_DEBUG_WINDOW") == 1) {
+        auto dv = new Debug::DebugWindow();
+        dv->show();
+    }
+#endif
+}
+
+Platform_qtwidgets::~Platform_qtwidgets()
+{
+}
+
+const char *Platform_qtwidgets::name() const
+{
+    return "qtwidgets";
+}