community/pipewire: add .desktop file to /etc/xdg/autostart
This makes Pipewire autostart on graphical sessions and is required to have audio if using as a PulseAudio replacement, and to get window previews with Plasma 5.21 on Wayland. Systemd based systems use socket activation but obviously that is not an option for us.
PulseAudio also uses this method to autolaunch so this should be fine