[Etoys] Revert the page bug in Demon Castle

[Takashi Yamamiya]

Hi Scott,

I found a bug about Book's revert feature.

- Open the Demon Castle project.
- Go to third page.
- Drag the key and show the door.
- Press "Revert the page" button.
- The screen is reverted but never works again.

Also 4th page doesn't work (explanation of right buton). I guess
someone saved the project after use "Revert the page" at 4th page.

Cheers,
- Takashi