Reading System Preferences and components should take the absence of PositionsList into account

[JayPanoz]

For components such as Progression, we’re using the PositionsList (with fallbacks in case it’s not available, see #9). In the future, developers will be able to pick the representation they want, and expose it to users as well (e.g. tapping the progression toggles the representation or additional details) so the dedicated preferences should take that into account.

This may impact the table of contents as well (see #38), and other components and their preferences.