PDF Scrapper: Restart ORB PDF Scrapper from a text or BibTeX source.

[j-steinbach]

Using the PDF Scrapper often involves manual cleaning of the extracted references in the "Text mode" and "BibTeX mode" buffer.

If we take the result of those buffers as self-contained steps (see #140), then the PDF Scrapper process should also be restart-able from those steps.

This removes the need to re-do the manual work again and also accommodates changes the user makes in their work-flow later on (for example changing their cite-key scheme or styling).