Spurious key signature on key change if mmrest starts a system (3.0.x only)

This relates to #280390: Spurious key change after transposition but is different.

1) load attached score
2) press "M" to enable mmrests
3) drag a new signature to bar 1
4) remove the line break after bar 4

Result: the old key signature remains at the start of the mmrest

This is because when we create the mmest, we are not using the fourth "addEvtToStaff" parameter to the ChangeKeySig command, so it's always getting marked as a true key change. It's the story as when using the Transpose command as per #280390: Spurious key change after transposition, but here the trigger is different and so is the place where it needs to be fixed. Just about done with a fix for both.