Display effort stats and meters based on classes.

[DroneEnthusiast]

This makes the appearance of effort on character sheets dependent on class. If the character in question has two classes with effort, two meters will appear. Although, with the core rules, this isn't useful, it can be used for Adventurers with custom partial classes which include effort.

This PR includes #35, #38, #56, #57, #58 and #59.