Add document property support

Use QObject::setProperty() to do so

examples/xlsx/documentproperty/documentproperty.pro

@@ -0,0 +1,7 @@
+TARGET = ducumentproperty
+
+#include(../../../src/xlsx/qtxlsx.pri)
+QT+=xlsx
+
+SOURCES += main.cpp
+

examples/xlsx/documentproperty/main.cpp

@@ -0,0 +1,28 @@
+#include <QtCore>
+#include "xlsxworkbook.h"
+
+#ifdef Q_OS_MAC
+#  define DATA_PATH "../../../"
+#else
+#  define DATA_PATH "./"
+#endif
+
+int main()
+{
+    QXlsx::Workbook workbook;
+/*
+    These properties are visible when you use the
+    Office Button -> Prepare -> Properties option in Excel and are also
+    available to external applications that read or index windows files
+*/
+    workbook.setProperty("title", "This is an example spreadsheet");
+    workbook.setProperty("subject", "With document properties");
+    workbook.setProperty("creator", "Debao Zhang");
+    workbook.setProperty("company", "HMICN");
+    workbook.setProperty("category", "Example spreadsheets");
+    workbook.setProperty("keywords", "Sample, Example, Properties");
+    workbook.setProperty("description", "Created with Qt Xlsx");
+
+    workbook.save(DATA_PATH"Test.xlsx");
+    return 0;
+}

examples/xlsx/xlsx.pro

@@ -1,3 +1,4 @@
 TEMPLATE = subdirs
-SUBDIRS = hello style
+SUBDIRS = hello style \
+    documentproperty