Rename undo-thunks to clear-thunks.
Rename undo-thunks to clear-thunks.

They don't have anything to do with editors' undo.

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