API: Deprecate key snooping

You really shouldn't be doing this. If you have to, you can use
gdk_window_add_filter() or listen to raw events on XI2.

gtk/gtkmain.h

@@ -152,8 +152,10 @@ void       gtk_device_grab_add     (GtkWidget          *widget,
 void       gtk_device_grab_remove  (GtkWidget          *widget,
                                     GdkDevice          *device);
 
+G_GNUC_DEPRECATED
 guint      gtk_key_snooper_install (GtkKeySnoopFunc snooper,
                                     gpointer        func_data);
+G_GNUC_DEPRECATED
 void       gtk_key_snooper_remove  (guint           snooper_handler_id);
 
 GdkEvent * gtk_get_current_event        (void);