Refactor BackendManager

[jorg-vr]

The goal would be to make the BackendManager part of Papyros instead of a global static class.
This would make the api more consistent with the other refactored code since #878

In ideal circumstances this could even allow hosting multiple papyros instances in the same session.

• Investigate if we can make this a subset of papyros instead of global
• Events should only be used for backend to frontend communication
• Serviceworker: test with multiple papyros instances
• Validate if we still need the custom flushing behaviour