patch 8.1.1321: no docs or tests for listener functions

Problem:    No docs or tests for listener functions.
Solution:   Add help and tests for listener_add() and listener_remove().
            Invoke the callbacks before redrawing.

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1321,
 /**/
     1320,
 /**/