Replace callbacks and local events with emitted events

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

Using events instead of callbacks to trigger behavior guarantees that components will have mutable access, and that the TUI will be rerendered after an update. The emitter trait gives each component that can emit events a unique ID, so we can't mix up events of the same type from different instances. It also ensures each component emits events of a single known type, so we get better type coercion from the event trait objects.

In some ways this adds complexity because we end up doing multiple rounds of event handling, which led to some bugs I had to fix. I think overall it's an improvement though, because it's more consistent.

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

This is a major refactor, so there's tons of risk of tiny bugs showing up related to UI events. Existing tests mitigate this a bit, but I'm going to let it soak on master for a bit and use it myself to see if I notice anything.

QA

How did you test this?

Lots of existing tests that I updated

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked