levels/world2: Repurpose "A Ghostly World"

"A Ghostly World", now a redesigned version of "Walking Flames" is
supposed to be the final level of the Ghost Forest. In addition, it
features a new type of secret area, where the player has to destroy
blocks using a bomb in order to enter it.

Closes SuperTux/data#68 ("levels/world2: Add ghostly_world.stl")
Reviewed-by: Max Teufel <max@teufelsnetz.com>
Tested-by: Max Teufel <max@teufelsnetz.com>

Author: Rusty-Box