Port the OpenPGP applet to GTK+ 3
_Originally created by @intrigeri on [#7711 (Redmine)](https://public-redmine-archive.tails.boum.org/code/issues/7711)_
The code currently lives at
<https://git-tails.immerda.ch/nodens/openpgp-applet/>. Work should be
done in the master branch first. Updating the upstream metadata
(dependencies in `META.yml` and/or `Makefile.PL`, etc.) would be
appreciated. Testing on current Debian stable *and* unstable is
important.
Feature Branch: nodens:feature/7711-Gtk3
### Related issues
- **Blocks** tails/tails#8310
- **Blocks** tails/tails#5967
- [x] **Blocked by** tails/tails#10190
- **Blocks** tails/tails#11899
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