daemon: Rely on systemd unit file for X11 plugin initialization

For GNOME X11 sessions, avoid relying on xdg autostart desktop files
to initialize the X11 plugins. This is now handled via a systemd unit
file.

The xdg autostart is still installed, but has been made to instruct
GNOME to skip it with X-GNOME-HiddenUnderSystemd. This is still the
primary way to initialize X11 plugins for other DEs.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/467>

src/daemon/systemd/user/pulseaudio-x11.service.in

@@ -0,0 +1,18 @@
+[Unit]
+Description=Sound Service (X11 Plugins)
+Requires=pulseaudio.service
+After=pulseaudio.service
+ConditionUser=!root
+PartOf=gnome-session-x11.target
+
+[Service]
+ExecStart=@PA_X11_BINARY@
+ExecStop=@PA_X11_BINARY@ stop
+LockPersonality=yes
+MemoryDenyWriteExecute=yes
+NoNewPrivileges=yes
+RestrictNamespaces=yes
+SystemCallArchitectures=native
+SystemCallFilter=@system-service
+Type=simple
+UMask=0077