fix: Ensure smooth synth ending

[Danielku15]

Issues

Fixes #1599

Proposed changes

We currently have a hard stop when we reach the end of the song. This leads to a cut-off on the samples and results in weird clicking noises. This PR changes the stop sequence of the synthesizer to:

1. Request a fast stop of all voices being played (not instantly)
2. Waiting for all voices to really stop providing samples before finishing

This way we have a smooth ramp-down of the audio (still fast though).

Checklist

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

Further details

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