This is a follow-up to #30450 so that _global_ `title_bar` configs
shadow _defaults_. The way `SettingsSources::json_merge` works is by
considering non-json-nulls as values to propagate. So it's important
that configs be `Option<T>` so any intent in overriding values is
captured.

This PR follows the same `*Settings<FileContent = *SettingsContent>`
pattern used throughout to keep the `Option`s in the "settings content"
type with the finalized values in the "settings" type.

Release Notes:

- N/A