FR: add tray icon

[minionflo]

It would be nice to have a tray icon when the app is running in the background to indicate that the app is indeed running and to stop it.

[ranfdev]

The app requests to run in the background by using the standard freedesktop interface: https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Background.html

Modern desktop environments should be compatible with this standard and show the background apps somewhere in the tray. At least GNOME does this.