racket/gui: fix problem with recent edit-sequence change
racket/gui: fix problem with recent edit-sequence change

Restores undo support in DrRacket, for example.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket