Placement of lines set by "X" command lost on save/reload

1) new score
2) add pedal line
3) use "X" command to flip above staff
4) save
5) close
6) reload

Result: line is back below the staff

See #278389: Some elements flipped via "X" command do not survive save/reload. This was fixed for some elements not too long ago. The fix probably should have worked as is, but the property flag parameter to undoChangeProperty is apparently not being passed to the property delegate. Eventually we should probably look at that, but who knows what side effect there would be from changing that, so I plan to make a safer fix for now, just setting the spanner's UNSTYLED flag right within the cmdFlip().