Nautilus uses NautilusPreviewer regardless of if we are using the development or default profile. Now that Sushi has introduced a nightly version, let's default the Nautilus nightly version to use Sushi nightly when available. If NautilusPreviewerDevel is not available then fall back to NautilusPreviewer
With these changes, we get a few freebies:
- No blocking i/o during application initialization
- Add a cancellable just in case
While we are at it, stop using the name "v2" since NautilusPreviewer2 is assumed to be used.
Depends on: sushi!40 (merged)