patch 8.1.1522: poup_notification() not implemented yet

Problem:    Popup_notification() not implemented yet.
Solution:   Implement it.
diff --git a/src/structs.h b/src/structs.h
index 8e6d93f..1fd4309 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1992,8 +1992,9 @@
     POPPOS_CENTER
 } poppos_T;
 
-# define POPUPWIN_DEFAULT_ZINDEX    50
-# define POPUPMENU_ZINDEX	    100
+# define POPUPWIN_DEFAULT_ZINDEX	 50
+# define POPUPMENU_ZINDEX		100
+# define POPUPWIN_NOTIFICATION_ZINDEX   200
 #endif
 
 /*