Finalmente, resolvi esse problema. O truque é usar a interface gsettings e os auxiliares de área de trabalho do snapcraft do Wiki ( desktop / gtk3 ). Estes são os arquivos principais. Eu os publiquei apenas no caso de serem úteis para outros resolverem um problema similar.
snapcraft.yaml
name: wallpaperdownloader
version: "2.2"
summary: Download and manage your favorite wallpapers from the Internet
description: WallpaperDownloader is a simple GUI Java based application for downloading and managing wallpapers from the Internet
grade: stable
confinement: strict
apps:
wallpaperdownloader:
command: wallpaperdownloader.sh
plugs: [x11, network-bind, home, gsettings]
parts:
# Pulls the code from the original source (master branch)
# desktop/gtk3 is a snapcraft part (snapcraft-desktop-helpers) from the Wiki: https://wiki.ubuntu.com/snapcraft/parts
# It enables desktop integration and gsettings manipulation from the confined application
# It is necessary to use gsettings interface (see above) in order to have a fully functional
# desktop/gtk3 part
# Github repository for snapcraft-desktop-helpers: https://github.com/ubuntu/snapcraft-desktop-helpers
wallpaperdownloader:
plugin: maven
source: ..
after: [desktop/gtk3]
# It will copy wallpaperdownloader script into /bin/
# This script contains all the commands needed (sets env variables, launches the jar file...) to
# execute the application
exec:
plugin: dump
source: scripts
wallpaperdownloader.sh
#!/bin/sh
# Only for packaging!
# Script for snap packaging wallpaperdownloader application. It is not related to the code itself
# Not good, needed for fontconfig
export XDG_DATA_HOME=$SNAP/usr/share
# Font Config
export FONTCONFIG_PATH=$SNAP/etc/fonts/config.d
export FONTCONFIG_FILE=$SNAP/etc/fonts/fonts.conf
export HOME=$SNAP_USER_DATA
desktop-launch java -jar -Duser.home=$SNAP_USER_DATA $SNAP/jar/wallpaperdownloader.jar