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/FrameWidget_p.h

@@ -0,0 +1,47 @@
+/*
+  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_FRAME_WIDGET_P_H
+#define KD_FRAME_WIDGET_P_H
+
+#include "Frame_p.h"
+
+class QTabBar;
+
+namespace KDDockWidgets {
+
+/**
+ * @brief The GUI counterpart of Frame. Inherits Frame and implements paintEvent().
+ */
+class DOCKS_EXPORT FrameWidget : public Frame
+{
+    Q_OBJECT
+public:
+    explicit FrameWidget(QWidget *parent = nullptr, FrameOptions = FrameOption_None);
+    QTabBar *tabBar() const;
+protected:
+    void paintEvent(QPaintEvent *) override;
+};
+
+
+}
+
+#endif