Improve attachments for QPdfWriter

In the European Union we have to embed the digital invoices (XML file) and other documents in a PDF. therefore the attachment has to get some flags.

In QPdfWriter::addFileAttachment() there are only 3 parameters:

QString fileName
QByteArray data
QString mimeType // optional

This creates the following items in the PDF Content:

/F (fileName)
/Subtype/mimeType (e.g. /Subtype/text#2Fxml)

But it is also required that the PDF flag

/AFRelationship /Alternative

is set to tell the system, that PDF and XML rely to the same content (alternative representation).

Also a description should be an optional parameter because this is what all other attachment programs can set.