Fix #20703 - Key sig changes lack naturals (1st solution)

Fix #20703 - Key sig changes lack naturals

When a key sig changes, no natural is ever displayed, regardless of the "Hide naturals" property of the key sig.

Corrected in undo.cpp (Score::undoChangeKeySig() ) by looking at the previous key sig and setting the accidental delta, when adding a new key sig.

Note: this fix does not cover updating naturals in following key sigs, if a previous key changes. This should probably dealt with during re-layout, rather than when adding/deleting key sigs.