This adds the JobManagerInterface helper object to emit a signal when a job is
started. It also renames the job prefix to be its own path so that the
JobManagerInterface can claim that as an object.
Rename ProcessManager to JobManager, as that's what it really handles. This
also renames get_command_object_path, which actually spawns a job instead of
just getting something. Further, it moves job spawning into the Job object
instead of being a helper function on ProcessManager.
