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.
From feedback from Andres changed SubProcess to just Job for
possible further changes to have the manager do things itself
without launching subprocesses.
Changed names objectpath to jobpath, exit_code to result, etc.
Removed Kill and added a force parameter to Cancel instead.
Also ran cargo fmt which tweaked indentation in a couple places.
Alos removed ExitCode, users can just call Wait again if needed.