Revise ThumbnailGeneration task
Revise ThumbnailGeneration task

- Gracefully shutdown on application close
- Prevent exception on application close
- Simplify looping logic
- Only WaitOne if thumbnail queue is empty

unlimitedbacon/MatterControl,larsbrubaker/MatterControl,larsbrubaker/MatterControl,jlewin/MatterControl,mmoening/MatterControl,jlewin/MatterControl,mmoening/MatterControl,jlewin/MatterControl,mmoening/MatterControl,unlimitedbacon/MatterControl,larsbrubaker/MatterControl,unlimitedbacon/MatterControl,unlimitedbacon/MatterControl,larsbrubaker/MatterControl,jlewin/MatterControl