Changes

Expose Space#setBackgroundTaskProgress

Space#setBackgroundTask isn't exposed yet, but that's mainly
because I intend to use the progress update logic in faststar
and fast-asynchronous-starlane-triangulation, where as
the background task description would only need to be set in
faststar, which can already access said method via gslStarplane.
 (commit: 56e2d9c)