Mechanism to allow centering on bars during scrolling

[Danielku15]

Based on #2046

alphaTab currently supports following scrolling modes:

1. Continuous

• on horizontal layouts: If the expected X-coordinate is changing (typically on bar changes).
• on vertical layouts: If the expected Y-coordinate is changing (typically on system changes).

2. OffScreen

• on horizontal layouts: If the expected X-coordinate goes out of the visible viewport.
• on vertical layouts: If the expected Y-coordinate goes out of the visible viewport.

For each of the respective layout types a fixed pixel offset can be configured which is applied.

As indicated in the discussion, there is a wish for rather centering the bar in the view port. alphaTab doesn't provide a good mechanism to handle this. We should think of proper events or means of extension to allow customizing the scrolling.