Fix #20703 - Key sig changes lack naturals (2nd 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 KeySig::layout() by looking at the previous key sig and setting the accidental delta.

Notes:
1) Covers updating naturals in following key sigs, if a previous key changes.
2) The fix is already predisposed to cope with a score style setting for display style of naturals: none, before, after accidentals. Until this style is implemented, the relevant code is commented out. For a discussion on 'styles' for positions of naturals, see: http://musescore.org/en/node/20724 .