ScreenShield: send a signal to GSD to wake up the screen

Instead of poking through IDLETIME, which confuses the state tracking
and can prevent automatic suspend, send a special signal to GSD
when the screen is to be waken up for a notification.

Someday we'll bring over all the state tracking and avoid this
ping-pong between gnome-shell and gnome-settings-daemon, but
that day's not today.

https://bugzilla.gnome.org/show_bug.cgi?id=712706

src/shell-util.h

@@ -41,8 +41,6 @@ GdkPixbuf *shell_util_create_pixbuf_from_data (const guchar      *data,
                                                int                height,
                                                int                rowstride);
 
-void    shell_util_wake_up_screen             (void);
-
 void    shell_util_cursor_tracker_to_clutter (MetaCursorTracker *tracker,
                                               ClutterTexture    *texture);