Add full portal support

[sergio-costas]

Due to some changes in GTK and GDK, GDK_DEBUG=portals is required in the environment variables to ensure the use of portals.

More info: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/4829

Pull Request Template

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[sergio-costas]

@kenvandine Or maybe we should ask James... (but its id doesn't show in github, don't know why)

[sergio-costas]

More info: https://forum.snapcraft.io/t/doesnt-use-the-xdg-desktop-portal/34360