Ensure proper plugin shutdown

From the [Eclipse Jobs docs](http://help.eclipse.org/neon/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Fguide%2Fruntime_jobs.htm):

> ### Completing jobs before shutdown
> Since jobs run concurrently, it is possible for your jobs to be still running when the platform begins to shutdown. This is a dangerous situation, since after your plug-in stops it may not behave correctly or be able to load classes. For this reason, it is important for you to ensure all your jobs are canceled and completed in your plug-in's stop method. As in the previous example, you can use a job family to ensure all jobs scheduled by your plug-in are canceled and joined before your plug-in stops running.