levels/world2: Add "This Forest is Haunted"

"This Forest is Haunted" is an additional new Ghost Forest level. It
features a second exit hidden in a secret area.
It can be used as a bonus level or in a separate Ghost Forest path on
the worldmap.

Ref: #69 ("levels/world2: Add "Bye Bye Forest" + "This
Forest Is Haunted"")
[[email protected]: taken from 31a2a45,
split off "This Forest is Haunted", and extended commit message]
Reviewed-by: Max Teufel <[email protected]>
Tested-by: Max Teufel <[email protected]>
Cc: alex jockers <[email protected]>
Cc: Max Teufel <[email protected]>
Cc: Carsten Wirtz <[email protected]>