Stub FocusSkillScore + Refactor

Skill scores don't have passive modifiers, they have passive values which
requires the attribute passive modifier. Pass the character to the score
to get the correct attribute for the skill using new method
Character.GetAttributeScore, then throw away the character because we just
need the score. I thought about taking the attribute score directly, but
that would allow the attribute score and the skill to be based on
different attributes. This way constrains the data so that can't happen.