Introduce QWidgetAdapter and apply it to MultiSplitter

MultiSplitter is now QWidget agnostic, and should work with
QtQuick

src/QWidgetAdapter.h

@@ -0,0 +1,56 @@
+/*
+  This file is part of KDDockWidgets.
+
+  Copyright (C) 2019 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com
+  Author: Sérgio Martins <sergio.martins@kdab.com>
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 2 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+/**
+ * @file
+ * @brief A class that is QWidget when building for QtWidgets, and QObject when building for QtQuick.
+ *
+ * Allows to have the same code base supporting both stacks.
+ *
+ * @author Sérgio Martins \<sergio.martins@kdab.com\>
+ */
+
+#ifndef KDDOCKWIDGETS_QWIDGETADAPTER_H
+#define KDDOCKWIDGETS_QWIDGETADAPTER_H
+
+#include "docks_export.h"
+
+#include <QWidget>
+
+namespace KDDockWidgets {
+
+class DOCKS_EXPORT QWidgetAdapter : public QWidget // TODO ifdef the base class once we support QtQuick
+{
+    Q_OBJECT
+public:
+    explicit QWidgetAdapter(QWidget *parent = nullptr);
+    ~QWidgetAdapter() override;
+
+protected:
+    bool event(QEvent *e) override;
+    void resizeEvent(QResizeEvent *) override;
+
+    virtual bool onResize(QSize oldSize, QSize newSize);
+    virtual void onLayoutRequest();
+};
+
+}
+
+#endif