main, util: Notify systemd once we are fully initialised

If graphical applications want to start from systemd units, they need to
start after we're properly ready to display them. This is particularly
important under X where `_GTK_FRAME_EXTENTS` and other xprops are needed
to have the right theming.

We're doing this in an idle callback so that the dynamic starting of
`gnome-session-x11-service.target` (which launches `gsd-xsettings`) as
the result of a signal emission happens before us signalling we're ready
for later things to start.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/750

js/ui/main.js

@@ -229,7 +229,11 @@ function _initializeUI() {
     EndSessionDialog.init();
 
     // We're ready for the session manager to move to the next phase
-    Meta.register_with_session();
+    GLib.idle_add(GLib.PRIORITY_DEFAULT, () => {
+        Shell.util_sd_notify();
+        Meta.register_with_session();
+        return GLib.SOURCE_REMOVE;
+    });
 
     _startDate = new Date();