fix #32651: Key signature at system break with canceling naturals should not repeat naturals at beginning of following system

Had to undo a separate bug's commits - I guess you shouldn't git rebase master if you're working on multiple bugs.
This change will force key signatures to never show naturals if they are at the system header, even if it's a change to C/Am. We could make this a style option, but this should be the default behavior.
I suspect simply testing for `measure()->systemHeader()` is not enough:

1. if there is a single measure for system, the same measure may both be at header and have a courtesy key sig. at the end: the latter will be drawn without naturals;

2. even at system beginning, if there is no courtesy key sig. before, I believe the (main) key sig. should display the naturals.