Customize display of bar numbers.

[Danielku15]

See #2386

The bar numbers in alphaTab are currently a fixed thing. They are rendered with the given fonts and colors.

Following functionalities are missing:

1. Allow showing / hiding of barnumbers for specific bars
2. See MusicXML measure[implicit])
3. Allow more flexible control over displayed numbers (show generally, hide generally, show on first bar of system)
4. Guitar Pro has an option there: