Restructure Part 2

Implementing prototype "timeline" controller, which will allow to select your preferred Game Mode when installing.

This also splits off the backend mods into a "core" modlist, where everything else is treated as effectively optional.

Mod dependencies are more strictly defined in the pack list to avoid issues caused by dependency mods being disabled, blocking dependent mods from being used.

This has been saved to a placeholder file as this setup currently doesn't work with ATL and will require a backend modification to become functional.