Remove finished ToDo
Fix exception on shut down
Delete IOperation.Stop(bool)

Don't want to deal with two mostly identical methods
Rename IOperation.Stop's 'deleteUnfinishedFiles' parameter to 'cleanup'
DownloadOperation: Remove Stop(bool), fix...

'Remove' context menu item related bugs, should work regardless of
status now.

Also renames 'deleteUnfinishedFiles' parameter to 'cleanup'.
Remove Stop(bool) from remaining operations