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

src/shell-util.h

@@ -66,6 +66,8 @@ gboolean shell_util_stop_systemd_unit  (const char  *unit,
                                         const char  *mode,
                                         GError     **error);
 
+void shell_util_sd_notify (void);
+
 G_END_DECLS
 
 #endif /* __SHELL_UTIL_H__ */