Notify attached UIs whenever menus change

This adds a redraw notification "update_menu" which is sent whenever
Vim's menus are changed by the :menu command and friends.

src/nvim/menu.c

@@ -30,6 +30,7 @@
 #include "nvim/keymap.h"
 #include "nvim/garray.h"
 #include "nvim/strings.h"
+#include "nvim/ui.h"
 
 
 #define MENUDEPTH   10          /* maximum depth of menus */
@@ -249,6 +250,7 @@ ex_menu (
     xfree(map_buf);
   }
 
+  ui_update_menu();
 
 theend:
   ;