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.
This adds a Parcellite service. It has no configuration options, since
the app has its own mutable preferences dialog, which unconditionally
replaces `~/.config/parcellite/parcelliterc` when preferences are
saved.
