enhancement: embedded files

[pr-apes]

@Frooodle,

it would be great to have attachments (embedded files) as a feature in Stirling-PDF.

To make them easily accessible, there are two options:

1. /PageMode /UseAttachments entry in the catalog dictionary will open the attachments pane.

2. Links to attachments.

The second option is the one used in this sample.

It is more intuitive for the end user to have links (although this option is only supported by PDF.js and Acrobat).

This being said, if the resulting main document should not contain new text (to include links), the first option is the only way to go.

Many thanks for your help.