Make needed Bravura stylistic alternates available to score font.

In ScoreFont::load() some ad-hoc code is added to set the proper codepoint for glpyphs only available as stylistic alternates.

As stylistic alternate encoding is not documented in Bravura .json files (and generally speaking in SMuFL), this code will need manual adjustments, should the stylistic alternate encoding change. However, all future additions and changes could be managed from this single code location.

At the moment, only the sixStringTabClefSerif symbol seems needed.

Note: the Bravura glyph for uniE06C.salt02 (= 6stringTabClefSerif) seems vertically misplaced.
Use metadata glyphWithAlternates section.
Merge pull request #940 from mgavioli/Bravura_stylistic_alternates

Make needed Bravura stylistic alternates available to score font.