Add attachments when creating a pdf

Add a method to QPdfWriter to add attachments to a pdf.

public void addFileAttachment(const QString& fileName, const QString& name,  const QByteArray& data, const QMap<QString, QString>& properties)

The implementation can be based on code in QPdfEnginePrivate::writeImage,

or a method QPdfEnginePrivate::writeObject(data, properties) could be created to be used when writing an image and when writing an attachment.