Tweak for SexLab Defeat: Tied-up victims invulnerable 1

This tweak modifies SexLab Defeat. When you tie-up someone who's been knocked down, they will be made essential and invulnerable until they are untied. This is to stop followers from killing someone you've just tied up.

There is one potential problem with this mod: if you tie-up someone who shares a base ID with another NPC (such as bandit leaders, and other generic NPCs), those other NPCs will also be made essential and invulnerable. I don't think there is any way to fix that, because the SetEssential() function only applies to an ActorBase, not an Actor. Let me know if I'm wrong.