patch 8.1.1138: plugins don't get notified when the popup menu changes

Problem:    Plugins don't get notified when the popup menu changes.
Solution:   Add the CompleteChanged event. (Andy Massimino. closes #4176)

src/autocmd.c

@@ -112,6 +112,7 @@ static struct event_name
     {"CmdUndefined",	EVENT_CMDUNDEFINED},
     {"ColorScheme",	EVENT_COLORSCHEME},
     {"ColorSchemePre",	EVENT_COLORSCHEMEPRE},
+    {"CompleteChanged",	EVENT_COMPLETECHANGED},
     {"CompleteDone",	EVENT_COMPLETEDONE},
     {"CursorHold",	EVENT_CURSORHOLD},
     {"CursorHoldI",	EVENT_CURSORHOLDI},
@@ -1794,6 +1795,17 @@ has_textyankpost(void)
 }
 #endif
 
+#if defined(FEAT_EVAL) || defined(PROTO)
+/*
+ * Return TRUE when there is a CompleteChanged autocommand defined.
+ */
+    int
+has_completechanged(void)
+{
+    return (first_autopat[(int)EVENT_COMPLETECHANGED] != NULL);
+}
+#endif
+
 /*
  * Execute autocommands for "event" and file name "fname".
  * Return TRUE if some commands were executed.