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.
The old method for hiding the error no longer works in NixOS 19.09,
and ends up breaking blueman-applet entirely. Enable the NixOS service
instead.
Pull request #950
