A cut-off's place in Plate.Parts is its cut sequence number, but
RegenerateCutOffs removed every cut-off part and appended it again, so
any part drag, fill or cut-off move sent the cut-offs to the end. The
nest file didn't store the position either, so reopening did the same.
RegenerateCutOffs now puts each cut-off back at its previous index (new
cut-offs go at the end), and CutOffDto.Sequence saves the index. Older
files without it load the cut-offs at the end, as before.
