-;;; Split the alist MENULIST into a nested alist, if it is long enough.
-;;; In any case, add TITLE to the front of the alist.
-;;; If IMENU--RESCAN-ITEM is present in MENULIST, it is moved to the
-;;; beginning of the returned alist.
-;;;
-;;; The returned alist DOES NOT share structure with MENULIST.