After a CopyJob the FileUndoManager records the file that was copied.
If this file is deleted before the Undo operation is triggered, the
File UndoManager will still try to undo the copy by deleting it.
This patch fixes this issue by validating the recorded files with
QFileInfo::exists(). If the FileUndoManager realizes that is no longer
possible to undo the CopyJob, it will signal that the undo operation is
not available anymore.