Add watermarking abilities

[almet]

What is the feature you think should be a good addition to Dangerzone?

I think it would be interesting to add support for watermarking. When you want to send sensitive documents, sometimes you want to mark them. I'm currently using pdf-watermark for it, like this:

uvx --with cryptography --from pdf-watermark watermark grid . "Only for Mister M.

This is a way to ensure that nobody else will use these documents for a different purpose.

Additional context

I believe this watermarking can be an issue when OCR is used, because it can trick the OCR engine in weird ways.

[almet]

I've just tried to run OCR on top of these documents with watermarked text in diagonal, and it works without issues (with both tesseract and macOS default OCR)