Centralized handlers

[pgivel]

Description

This PR introduces a mechanism of centralized handlers. This allows all of the handlers to be cleanely removed at destruction time, and for the device renderer to be fully picked up by the garbage collector.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [x] I've read & comply with the contributing guidelines
• [ ] I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• [x] I have made corresponding changes to the documentation (README.md).
• [x] I've checked my modifications for any breaking changes.