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.
Fix tests.

As test scores are in 1.14 format, they get an extra keySigNaturals = BEFORE style.

Updated reference scores.
Merge pull request #364 from mgavioli/Style_for_natural_pos_in_key_sig

Score style for natural position in key signatures