Add additional ways to add undead fortitude

[Flix]

Some monsters, especially homebrew ones, may have features that are functioncally identical to undead fortitude in everything but name. To support this the following change is to be implemented:
Instead of checking only the item names to do stuff also check for a flag on the actor. If either is existent execute normal functionality.
Add a tickbox in the special traits menu to allow adding said flag to actor via ui

(Probably going to do this myself at some point)