Add Ability.Skill constructor SkProtect Skill that protects an actor from status conditions draining the given skill

[Mikolaj]

Negative value of this skill would represent susceptibility and would increase the drain by the value (TODO for later, because this is currently hard, due to fixed status conditions defined in content; perhaps for now it can increase the count of the status condition copies obtained). Positive value would project from the drain up to the value and fail if the drain is larger (TODO: for now, it would make the whole status condition fizzle, because creating a new content item with only the skill drain in question removed is problematic ATM). UI is needed to let the player know that protection succeeded or failed.

This would probably require splitting Ability.Skill into the base skills (or, separately, stats and skills) and the special skills, such as SkProtect and the already present SkDeflectRanged (BTW, perhaps the letter should deflect up to a given threshold and represent vulnerability if negative?). Then we'd need to manually define the Enum instance.

Let's gather other properties of actors that don't fit the current system of integer skills to see how to extend it with lowest cost/benefit.

One such property is protection from Effect effects, but it's problematic, because effects don't have tags that denote them. An option would be to protect against all effects specific items instead (e.g., torches), such as currently actors can be resistant to poison (a specific status condition defined in content).