feat: smooth cursor scrolling.

[Danielku15]

Issues

Fixes #2371

Proposed changes

In its core this PR adds a new scroll mode Smooth which performs a smooth scrolling as described in the issue.

For horizontal layouts we have an almost static scroll following the beat cursor. Due to browser scrolling and animation inconsistencies there can be slight flickerings of the cursor. If somebody wants a 100% static cursor, it can be placed outside of alphaTab on top of the music sheet at the right spot. There are enough APIs available to collect the positions and update things when needed.

For vertical layouts we have a gradual scroll from the top of the system to the bottom over the duration of its playback.

Separately to that the whole scrolling logic can now be implemented via a custom IScrollHandler. If anyone want's to build a custom scrolling logic with alternative offsets or calculations, this interface can be used.

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• New tests were added

Further details

• This is a breaking change
• This change will require update of the documentation/website