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/ui.c

@@ -324,6 +324,11 @@ void ui_cursor_goto(int new_row, int new_col)
   pending_cursor_update = true;
 }
 
+void ui_update_menu(void)
+{
+    UI_CALL(update_menu);
+}
+
 int ui_current_row(void)
 {
   return row;