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/WidgetResizeHandler_p.h

@@ -0,0 +1,73 @@
+/*
+  This file is part of KDDockWidgets.
+
+  Copyright (C) 2018-2019 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/>.
+*/
+
+#ifndef KD_WIDGET_RESIZE_HANDLER_P_H
+#define KD_WIDGET_RESIZE_HANDLER_P_H
+
+#include <QWidget>
+#include <QPoint>
+#include <QDebug>
+
+class QMouseEvent;
+
+namespace KDDockWidgets {
+
+class FloatingWindow;
+
+class WidgetResizeHandler : public QObject
+{
+    Q_OBJECT
+public:
+    explicit WidgetResizeHandler(QWidget *target = nullptr);
+    ~WidgetResizeHandler() override;
+
+    void setTarget(QWidget *w);
+
+#ifdef Q_OS_WIN
+    static bool handleWindowsNativeEvent(FloatingWindow *w, const QByteArray &eventType, void *message, long *result);
+#endif
+    static bool s_disableAllHandlers;
+protected:
+    bool eventFilter(QObject *o, QEvent *e) override;
+
+private:
+    enum class CursorPosition {
+        Left,
+        Right,
+        TopLeft,
+        TopRight,
+        BottomRight,
+        BottomLeft,
+        Top,
+        Bottom,
+        Undefined
+    };
+    void mouseMoveEvent(QMouseEvent *e);
+    void updateCursor(CursorPosition m);
+    CursorPosition cursorPosition(QPoint) const;
+    QWidget *mTarget = nullptr;
+    CursorPosition mCursorPos = CursorPosition::Undefined;
+    QPoint mNewPosition;
+    bool mResizeWidget = false;
+};
+
+}
+
+#endif