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 new options allow some control over automounting, notifications,
and the tray icon.
This commit also changes the defaults to automatically mount new
devices, udiskie was previously told not to automount. The change in
behavior is to closer match the default options.
