Adjust root terminal launcher on Buster wrt. the gksu removal

In Tails 3.x we create a root terminal app launcher using the icon and desktop file from the gksu package (config/chroot_local-hooks/20-root_terminal)..) But gksu won’t be in Buster.

Presumably the easiest solution is to:

import the .desktop.in file into our Git repo, integrate it with refresh-translations and po/POTFILES.in, import existing translations into po/*.po
import the icon we’ve used so far into our Git repo

Feature Branch: bugfix/15335-replace-gksu-root-terminal-on-buster

Original created by @intrigeri on 15335 (Redmine)

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