Rewrite key selection interface

The public key selection interface in OpenPGP Applet currently uses Gtk3::SimpleList.

The problem is, since it's a simple list we can't display the ids as subentries, and we use a tooltip for that. Also, it's buggy since the port to Gtk3 (#7). By using the complete Gtk::Treeview implementation, we could:

• fix the bug
• display the ids as subentries of each public key
• add a search box

Ideally, we would mimic Seahorse's interface. The first implementation doesn't have to be as neat, though.

Copied from Tails bug tracker #13602