Code refactoring: Clean up code of QXlsx::Package

We should merge the class Package with the class Document

src/xlsx/xlsxcontenttypes.cpp

@@ -26,6 +26,7 @@
 #include <QXmlStreamWriter>
 #include <QFile>
 #include <QMapIterator>
+#include <QBuffer>
 
 namespace QXlsx {
 
@@ -111,6 +112,15 @@ void ContentTypes::addVbaProject()
     addOverride(QStringLiteral("bin"), QStringLiteral("application/vnd.ms-office.vbaProject"));
 }
 
+QByteArray ContentTypes::saveToXmlData() const
+{
+    QByteArray data;
+    QBuffer buffer(&data);
+    buffer.open(QIODevice::WriteOnly);
+    saveToXmlFile(&buffer);
+    return data;
+}
+
 void ContentTypes::saveToXmlFile(QIODevice *device) const
 {
     QXmlStreamWriter writer(device);