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/meson.build

@@ -20,11 +20,11 @@ desktop_daemon_file = i18n.merge_file(
   install_dir : join_paths(sysconfdir, 'xdg/autostart')
 )
 
-# DBus service
 service_data = configuration_data()
 service_data.set('bindir', full_bindir)
-service_file = 'org.gnome.Calls.service'
 
+# DBus service
+service_file = 'org.gnome.Calls.service'
 configure_file(
   input : service_file + '.in',
   output : service_file,
@@ -33,6 +33,16 @@ configure_file(
   install_dir : full_servicedir,
 )
 
+# Systemd user service
+systemd_unit = 'calls-daemon.service'
+configure_file(
+  input : systemd_unit + '.in',
+  output : systemd_unit,
+  configuration : service_data,
+  install : true,
+  install_dir : systemd_user_unit_dir,
+)
+
 desktop_utils = find_program('desktop-file-validate', required: false)
 if desktop_utils.found()
   test('Validate desktop file',