fix(player): grace beats on song start

[Danielku15]

Issues

Fixes #2224

Proposed changes

Adds support for having before-beats at the start of the song. To handle the negative ticks these beats causes we do these things:

1. We shift the generated midi by the needed ticks.
2. When mapping any info from the synthesizer to the alphaTab API level we apply the shift if needed
3. The player needs to respect this additional shift when generating metronome ticks.

This way on API level everything stays negative and in-line with the data model. But the player can shift things.

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