data: Flag in desktop file that gnome-shell can be started using systemd

As gnome-shell is a required component for GNOME sessions, gnome-session
will currently always try to autostart it. However as we are moving towards
using systemd's user instance for session startup, gnome-session should only
be responsible for launching the shell when either not running under systemd
or when we were built without systemd support.

gnome-session can detect the former but not the latter, so communicate this
via the newly added X-GNOME-HiddenUnderSystemd key in the .desktop file.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/507
https://gitlab.gnome.org/GNOME/gnome-session/merge_requests/13

data/meson.build

@@ -14,6 +14,8 @@ desktopconf = configuration_data()
 # file when built in a non-system prefix
 desktopconf.set('bindir', bindir)
 desktopconf.set('VERSION', meson.project_version())
+desktopconf.set('systemd_hidden', have_systemd ? 'true' : 'false')
+
 foreach desktop_file : desktop_files
   i18n.merge_file('desktop',
     input: configure_file(

data/org.gnome.Shell.desktop.in.in

@@ -14,3 +14,4 @@ X-GNOME-Autostart-Phase=DisplayServer
 X-GNOME-Provides=panel;windowmanager;
 X-GNOME-Autostart-Notify=true
 X-GNOME-AutoRestart=false
+X-GNOME-HiddenUnderSystemd=@systemd_hidden@