feat: Multi System Slurs

[Danielku15]

Issues

Fixes #2261

Proposed changes

Adds support for multi-system slurs. After some rounds attempting to optimize some bits, the final result is not much different from the old logic:

• We have start ties/slurs and end ties/slurs created for the respective start/end notes or beats.

Additionally we have now:

• The start ties register themselves on layout level if multi-system tie/slur is detected.
• The end ties mark the end of the multi-system ties.
• During layout finalization any "ongoing" ties are added to the first bar of the system, layouted and rendered like normal ties.

The continuation ties are simply on the height of the tie at the starting system. No special collision detection or shifting exists for now. This would be a more general topic to address.

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