data: add systemd unit file for running daemon

This adds a systemd unit file for running the Calls daemon, and adds a
key to the xdg autostart file to instruct gnome-session to ignore
starting when systemd is present. The expectation is that systemd
distros will enable the Calls daemon as a user service and not rely on
xdg autostart / gnome-session for running/managing the service. Systemd
is better at managing services than gnome-session.

Note: it's important to keep the xdg-autostart file around, some distros
(pmOS on openrc, others) don't use systemd and will still need to rely
on the xdg autostart mechanism for starting the Calls daemon.

Co-authored-by: Guido Günther <agx@sigxcpu.org>

data/org.gnome.Calls-daemon.desktop.in

@@ -15,3 +15,4 @@ Terminal=false
 Categories=Network;GNOME;GTK;Telephony;
 X-GNOME-AutoRestart=true
 X-GNOME-UsesNotifications=true
+X-GNOME-HiddenUnderSystemd=true