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/TabBarWidget_p.h

@@ -0,0 +1,56 @@
+/*
+  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/>.
+*/
+
+/**
+ * @file
+ * @brief Implements a QTabWidget derived class with support for docking and undocking
+ * KDockWidget::DockWidget as tabs .
+ *
+ * @author Sérgio Martins \<sergio.martins@kdab.com\>
+ */
+
+#ifndef KD_TABBAR_WIDGET_P_H
+#define KD_TABBAR_WIDGET_P_H
+
+#include "TabWidget_p.h"
+
+#include <QTabBar>
+
+class QMouseEvent;
+
+namespace KDDockWidgets {
+
+class DockWidget;
+class TabWidget;
+
+class DOCKS_EXPORT TabBarWidget : public QTabBar, public TabBar
+{
+    Q_OBJECT
+public:
+    explicit TabBarWidget(TabWidget *parent = nullptr);
+    int numDockWidgets() const override;
+    int tabAt(QPoint localPos) const override;
+protected:
+    void mousePressEvent(QMouseEvent *) override;
+
+};
+}
+
+#endif