Make background feature(s) come from item Advancements.

[ccjmk]

• Replace Background compendium handling to fetch Background types instead of Features.
• Add new Background Features compendium handling.
• Remove all the botched handling of "background" Feature-type items that parsed the background name from the Requirement
• Make background selector directly offer all Background type items fetched
• Fill background feature selector with all Feature type items fetched
• Auto-select feature based on Advancement when user selects a Background