Handle empty bars and pre-beat grace notes on new lookup logic

[Danielku15]

Issues

Fixes #1339

Proposed changes

Fixes the two scenarios described in the bug report:

• We need to accept the beat of empty voices in the lookup to have the cursor considered.
• We need to ensure we add pre-beat grace notes also to the previous bar lookup if possible
• We need to ensure we take the original beat playback start for the visibility check, not the slice start.

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