Port the OpenPGP applet to GTK+ 3

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 #8310
Blocks #5967 (closed)
Blocked by #10190 (closed)
Blocks #11899 (closed)

Original created by @intrigeri on 7711 (Redmine)

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