Class character sheet additions

[DroneEnthusiast]

This patch adds class tracking to character sheets.

In addition it ensures all class fields are modifiable, including descriptions and graphics so a player can create ad-hoc custom classes.

Follow up patches will be the following:

• integrate HP bonus per-level into hp roll
• display effort only if a character has it
• add section for tracking special class abilities
• add attack bonus per-level to character total

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