Score style for natural position in key signatures

Adds a score style for selecting the position of naturals in key signatures:

- NONE (except when changing to CMaj/Amin)
- BEFORE accidentals
- AFTER accidentals (but always before if sharps <=> flats)

Default is NONE.

When reading old scores (<= 1.14), a BEFORE setting (as in 1.3) is provided.