Support tuning alterations

[Danielku15]

#2785

Problem

Currently alphaTab forces the use of sharps on the string tuning display. It doesn't allow changing the tuning to prefer the display of flats. Guitar Pro also allows specifying the tuning alteration.

Desired behavior

alphaTab should allow customizing the exact display of tunings.

• The guitar pro input should be respected
• alphaTex needs to allow specifying the alteration too. Likely we should adopt the alteration as explicitly written by the author. That means the alteration should be per-string, not globally.
• On MusicXML the https://www.w3.org/2021/06/musicxml40/musicxml-reference/elements/tuning-alter/ might give an indication whether we have sharps or flats. Negative values are flats and positive ones sharps.