This target is for systemd units that require a system tray to be
running.
This also fixes taffybar.service: previously, systemd would consider it
to be active (running) before it was actually ready to accept tray
icons.
Adds a service for the Stalonetray system tray.
Configured through a 'config' attribute set, which writes space
separated key value pairs on successive lines to `~/.stalonetrayrc`.
