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/DropAreaWithCentralFrame.cpp

@@ -0,0 +1,43 @@
+/*
+  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/>.
+*/
+
+#include "DropAreaWithCentralFrame_p.h"
+#include "Config.h"
+#include "FrameworkWidgetFactory.h"
+
+using namespace KDDockWidgets;
+
+DropAreaWithCentralFrame::DropAreaWithCentralFrame(QWidgetOrQuick *parent, MainWindowOptions options)
+    : DropArea(parent)
+    , m_centralFrame(createCentralFrame(options))
+{
+    if (m_centralFrame)
+        m_layout->addWidget(m_centralFrame, KDDockWidgets::Location_OnTop, {});
+}
+
+DropAreaWithCentralFrame::~DropAreaWithCentralFrame()
+{
+}
+
+Frame* DropAreaWithCentralFrame::createCentralFrame(MainWindowOptions options)
+{
+    return (options & MainWindowOption_HasCentralFrame) ? Config::self().frameworkWidgetFactory()->createFrame(nullptr, FrameOptions() | FrameOption_IsCentralFrame | FrameOption_AlwaysShowsTabs)
+                                                        : nullptr;
+}