Get rid of libgtk3-simplelist-perl

The OpenPGP Applet currently relies on Gtk3::SimpleList (or will once feature/Gtk3 is merged on my repo)

Since the upstream doesn't look very active (one single release which pretty much consisted of s/Gtk2/Gtk3 without even updating the doc), this could cause maintenance problem in the future, so it's probably best to not rely on it.

The main impact is on the key display/selection.

While we're at it, we could try to mimic the Seahorse interface, to provide a more consistent UX.

Copied from Tails bug tracker #11746