Allow to use the FreeDesktop background portal API instead of KSNI, as KSNI is going to be considered deprecated

[marin-m]

Cf. https://gitlab.gnome.org/Teams/Circle/-/work_items/269#note_2837339

Cf. https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Background.html

Also turn the switch to enable the tray icon into a tri-state paired with a combo box so this is tunable correctly in the app preferences.

[marin-m]

See https://github.com/bilelmoussaoui/ashpd