React to events in the Viewer

We support registering callbacks to Viewer events in these environments:

• Web browsers, through our JS package
• Jupyter notebooks, through our Notebook API

For users extending the Viewer through the Rust re_viewer crate, there are two options:

• Use StartupOptions.on_event to register the same events available on the web and in Jupyter.
• Extend the UI to add your own widgets using egui, and fire completely custom events.