Add ability to export diagram as PNG

[qallpat]

I would like to be able to create thumbnails of diagrams to use e.g.:

In an open dialog box, with each diagram displayed as name, category and the thumbnail image.

Export to PNG scaled to specified max width would be ideal e.g.

diagram.Export(ImageFormat.Png, 300);

I would expect the diagram to take the extents of the content on the diagram, calculate width and height, then (if the width is larger than the specified max width), scale width and height down in proportion to the max width specified.