Split class DocProps to DocPropsCore and DocPropsApp

So that, one class can deal with only one xml file

src/xlsx/xlsxdocpropscore_p.h

@@ -0,0 +1,48 @@
+/****************************************************************************
+** Copyright (c) 2013 Debao Zhang <hello@debao.me>
+** All right reserved.
+**
+** Permission is hereby granted, free of charge, to any person obtaining
+** a copy of this software and associated documentation files (the
+** "Software"), to deal in the Software without restriction, including
+** without limitation the rights to use, copy, modify, merge, publish,
+** distribute, sublicense, and/or sell copies of the Software, and to
+** permit persons to whom the Software is furnished to do so, subject to
+** the following conditions:
+**
+** The above copyright notice and this permission notice shall be
+** included in all copies or substantial portions of the Software.
+**
+** THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+** EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+** MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+** NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+** LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+** OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+** WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+**
+****************************************************************************/
+#ifndef XLSXDOCPROPSCORE_H
+#define XLSXDOCPROPSCORE_H
+
+#include "xlsxglobal.h"
+#include <QObject>
+#include <QList>
+#include <QPair>
+#include <QStringList>
+
+class QIODevice;
+
+namespace QXlsx {
+
+class XLSX_AUTOTEST_EXPORT DocPropsCore : public QObject
+{
+    Q_OBJECT
+public:
+    explicit DocPropsCore(QObject *parent = 0);
+        
+    void saveToXmlFile(QIODevice *device);
+};
+
+}
+#endif // XLSXDOCPROPSCORE_H