feat: redact metadata #3

Description

Currently only the contents of the PDF file are redacted, but contents to be redacted may be hidden in metadata. It should be possible to redact also metadata like author, document title, etc.

Implementation

• Design options:
  1. as additional tab in the main window
  2. as additional dialog during saving
• If #2 is implemented, it should also look for occurences in the metadata to be redacted.

See also Paper Clip, a GTK4 app that can be used to modify pdf document metadata.

The default should probably be to sanitize and strip all metadata away. Check if Paper Clip may be used to add metadata, it could be instructed to use other tools to add back metadata.

Edit: Indeed, Paper Clip can be used to add metadata back. Thus, I think removing metadata is the best default.

Implemented in !140