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)
diff --git a/src/vim.h b/src/vim.h
index 01123e0..af779f3 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1270,6 +1270,7 @@
     EVENT_CMDWINLEAVE,		// before leaving the cmdline window
     EVENT_COLORSCHEME,		// after loading a colorscheme
     EVENT_COLORSCHEMEPRE,	// before loading a colorscheme
+    EVENT_COMPLETECHANGED,	// after completion popup menu changed
     EVENT_COMPLETEDONE,		// after finishing insert complete
     EVENT_CURSORHOLD,		// cursor in same position for a while
     EVENT_CURSORHOLDI,		// idem, in Insert mode