support file attachment annotations

[pamapa]

Ported sub-feature from this pull request: https://github.com/empira/PDFsharp/pull/11/files
Part of #237

Usage:

private static void AttachFile()
{
    string text = "AttachFile.pdf";
    XFont font = new XFont("Verdana", 10.0, XFontStyle.Regular);
    PdfDocument pdfDocument = new PdfDocument();
    PdfPage pdfPage = pdfDocument.AddPage();
    XGraphics xGraphics = XGraphics.FromPdfPage(pdfPage);
    xGraphics.DrawString("Open with Adobe Acrobat Reader and click on the icon", font, XBrushes.Black, new XRect(10.0, 100.0, 300.0, 10.0), XStringFormats.TopLeft);
    pdfPage.Annotations.Add(new PdfFileAttachmentAnnotation(pdfDocument)
    {
        Rectangle = new PdfRectangle(new XRect(10.0, pdfPage.Height - 100.0, 10.0, 10.0)),
        File = new PdfFileSpecification(pdfDocument, "logo.jpg", new PdfEmbeddedFile(pdfDocument, File.ReadAllBytes("..\\..\\resources\\logo.jpg"), null))
    });
    pdfDocument.Save(text);
    Process.Start(text);
}

[ststeiger]

Merged.