Remove usages of `require_relative` that can be handled by `autoload`

What does this Merge Request do and why?

This removes many usages of require_relative which slows down GDK load time and moves to the autoload version.

This changes affects "Settings API" part of the codebase

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.