refactor: change to worker pattern for all platforms

[Danielku15]

Issues

Fixes #2615, #2616, #2617, #2621, #2629

Proposed changes

The threading model of the .net and Android version were quite different to the single-thread nature of JavaScript (+1 thread per worker).

This can lead to race-conditions and concurrent modifications in the .net and Android threading model.

This PR refactors quite some parts there:

• We share now more code with the web version and only re-implement the worker threading model in .net and Android.
• We try to stick to a clean "main" (UI) and single thread per "worker" model.
• Skia was wrongly configured to be worker-incompatible causing rendering to be done in the UI thread.
• Some further fixes around cursor handling and state handling for .net and android should make things more safe.

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 (manual tests done, no API/control level automatic tests yet)

Further details

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