Implement [fade_in_sound_effects] WML action
E1S12: Various cutscene improvements

 * Delay set-up of the hive sound source until the player enters the
   boss chamber.
 * Make use of additional warp-scrolls, music fade-outs, sound
   fade-outs, sound fade-ins.
 * Visually fade-out Elynia during the end sequence in a manner similar
   to the units in E3S13.
 * Kill Elynia permanently at the end since we don't really need her to
   advance to E1S13.