Centered full bar notes and rests

[Danielku15]

See #2463

It is an established practice to center elements of bar-length durations within the bar. (Behind Bars page 41).

Would be nice if alphaTab can also render elements like this. This kind of display would only be active if we have a single beat in the bar with the full bar duration including all tracks/staves/voices rendered (on BarLayoutingInfo level).

If there is any inconsistency or multiple beats the classical positioning would be active.

Technical details: I think it could be as simple as checking for this condition here in buildOnTimePositions . If we have a single beat wth a full bar duration, we can shift the on-time position to the center and the rest should work out-of-the box.