Move the private code into a private/ folder

It's nice to have a similar include hierarchy at build time and install
time.

It also makes things more explicit

src/private/widgets/QWidgetAdapter_widgets_p.h

@@ -0,0 +1,72 @@
+/*
+  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_QWIDGETADAPTERWIDGETS_P_H
+#define KDDOCKWIDGETS_QWIDGETADAPTERWIDGETS_P_H
+
+#include "../../docks_export.h"
+
+#include <QWidget>
+
+namespace KDDockWidgets {
+
+class FloatingWindow;
+
+class DOCKS_EXPORT QWidgetAdapter : public QWidget
+{
+    Q_OBJECT
+public:
+    explicit QWidgetAdapter(QWidget *parent = nullptr, Qt::WindowFlags f = {});
+    ~QWidgetAdapter() override;
+
+    ///@brief returns the FloatingWindow this widget is in, otherwise nullptr
+    FloatingWindow *floatingWindow() const;
+
+    void setFlag(Qt::WindowType, bool on = true);
+
+protected:
+    void raiseAndActivate();
+    bool event(QEvent *e) override;
+    void resizeEvent(QResizeEvent *) override;
+    void mousePressEvent(QMouseEvent *) override;
+    void mouseMoveEvent(QMouseEvent *) override;
+    void mouseReleaseEvent(QMouseEvent *) override;
+    void closeEvent(QCloseEvent *) override;
+
+    virtual bool onResize(QSize newSize);
+    virtual void onLayoutRequest();
+    virtual void onMousePress();
+    virtual void onMouseMove(QPoint globalPos);
+    virtual void onMouseRelease();
+    virtual void onCloseEvent(QCloseEvent *);
+};
+
+}
+
+#endif