Clean up book mode transitions

- Always play the book turn sound when switching between pages,
  no matter how the page is changed.
- Prev/next key bindings can no longer switch to the spell page
  if the player has no runes. This reflects the behavior of the
  open spell page key binding.
- Fade out rune symbol draw flares when switching away from
  the spell page.

The rune fade out was previously implemented in arx-fatalis-fixed/r28

See: task #276