Settings Rework

[HJfod]

Reworks settings, with the following changes:

• New title type for creating collapsible titles
• Split path type into file and folder types
• Deprecate custom type, add ability for mods to add proper custom types via the custom:type-name syntax
• Settings have been simplified into a single SettingV3 class, which handles all the base properties like name and description
• Settings are now PIMPL'd in the new classes
• The SettingValueSetter class has been replaced with SettingTypeForValueType
• Old settings system is now deprecated, though is still fully backwards-compatibly API and ABI usable as SettingV3 is automatically translated back into the old system on request

This branch also has a bunch of other changes, all mentioned in the changelog.