Rework state handling of one-time-midi

[Danielku15]

Issues

Fixes #758

Proposed changes

Reworks quite some internals of the synthesizer to ensure that the one time midi playback behind playBeat and playBeat are working correctly. it suffered from following issues:

• Seeking during playback interfered with the playback position of the one time midi while it should seek on the main song midi. When playing notes/beats on mouse click, there happened a playback and seeking which messed up a lot of internals.

• Multiple subsequent requests of playing one time midis before they actually finished also messed up some internal states.

• The synth outputs reported also played samples if there weren't enough samples in the buffer. This wrongly advanced the internal player time axis.

• The bounds lookup had a wrong assumption that there is only one bounds for a beat, but there can be multiple, due to this findNoteAt did not work correctly.

• There was a wrong time value passed to the midi note generation of playNote leading the note to never being played.

• New: Events when a note is clicked. These events are available if the note bounds are included.

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