Move UniverseBackend to WebWorker

[BarthPaleologue]

The long term goal is to have a UniverseService async interface that can be implemented locally using a worker thread, or remotely using a server.

Such service would be async by nature and would provide additional features such as stellar path finding and system finding.

Proposed general shape:

UniverseService {
    SystemModelProvider,
    StellarPathFinder,
    SystemFinder
}

And the public interface would be a blend of the public interfaces of the different components.

This would also complete #139 at the same time.