TB 43186: Stop using `TorProviderBuilder` to prompt users on desktop.

Also replace `settledState` with `currentState` and `settled`.

TB 43186: TorConnect handles TorProvider initialisation errors.

+ Add the `"ProviderStopped"` stage, which we enter whenever the
  current `TorProvider` needs replacing.
+ In the `init` method we check `TorProvider.settledState` to see if the
  current provider is still running at startup. After this, we listen
  for the `ProviderStateChanged` event to see if it stops running later
  on.
+ We expose a `restartProvider` method to request a new provider. The
  `ProviderStatusChange` event allow consumers to update the UI.

TB 43186: Add a `"ProviderStopped"` page to about:torconnect.

TB 43186 - Add strings for the new `"ProviderStopped"` stage.