[3.13] 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
PulseAudio also uses this method to autolaunch so this should be fine
(cherry picked from commit a6257232)