Add IntersectionObserver for initial load and lazy loading

[Danielku15]

Issues

Fixes #510

Proposed changes

1. Replaces the old timer based visibility checker with the IntersectionObserver
2. Replaces the getBoundingClientRect based lazy loading system with the IntersectionObserver

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