Add document property support

Use QObject::setProperty() to do so

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;
+}