Add support for setting a job's status to ABORTING
Plug-ins that supports aborting may not always respond immediately. Partly due to things discussed in #892. While we are waiting for the plug-in to response it would be good to let the user know something about it. Currently the status information for a job will say 'Executing' until the plug-in has actually responded and exited with 'Error: Aborted by user'