Refactor OpenSimCreator/Graphics/* enum metadata functions

[adamkewley]

Broken out from #782

In order to implement settings support in #782, various methods were added that map graphics enums (e.g. MuscleColoringStyle) to metadata structs that contain serializable information like names, and ids.

Refactoring should:

• Try to name the helper functions more consistently
• Remove their dependence on reading from an unordered_map<std::string, AppSettingValue> and, instead, on something higher-level, like std::function<std::optional<AppSettingValue const&>(std::string_view)>
• (so that they are more generic, can be composed more cleanly into a hierarchy, etc.)