libview: add a method to get the job currently running in the worker thread

libview: add a method to get the job currently running in the worker thread

When a job is cancelled while it's running, the cancelled signal might be
emitted before the job finishes, and since the finished signal is not
emitted for cancelled jobs, it's not possible to know when the job has
finished. With this method we can see whether the job is still running
and wait until it finishes.