Add JS timeline control and callback APIs

[jprochazk]

Related

• Closes Time selection control javascript API #8329

What

Adds the following methods to WebViewer:

get_active_recording_id(): string | null;
set_active_recording_id(value: string): void;
get_playing(recording_id: string): boolean;
set_playing(recording_id: string, value: boolean): void;
get_current_time(recording_id: string, timeline: string): number;
set_current_time(recording_id: string, timeline: string, time: number): void;
get_active_timeline(recording_id: string): string | null;
set_active_timeline(recording_id: string, timeline: string): void;
get_time_range(recording_id: string, timeline: string): { min: number; max: number; } | null;

As well as the following events:

interface Events {
  timelinechange: [timeline: string, time: number];
  timeupdate: number;
  play: void;
  pause: void;
}

The event names are somewhat inspired by HTMLMediaElement, e.g. timeupdate.

See https://github.com/rerun-io/web-viewer-example/blob/jan/temp-timeline-controls-example/src/main.ts#L15 for an example usage of these new APIs.

---

For the time being, I only added what I thought was necessary to implement an "external timeline". I ended up making all the new methods very explicit in which recording and timeline they deal with, but it's not too verbose to work with. I don't feel strongly about any of the names, they are fully open to bikeshedding 🙂.

All event callbacks are called from TimeControl::update. Time updates are not throttled in any way, but they don't seem to be frequent enough to show up in any profiles or noticeably slow down the viewer. That is, unless you put a lot of JS code in them.

[github-actions]

Web viewer built successfully. If applicable, you should also test it:

• I have tested the web viewer

Result	Commit	Link	Manifest
✅	c160fc7	https://rerun.io/viewer/pr/8673	+nightly +main

^{Note: This comment is updated whenever you push a commit.}

[jprochazk]

(drafted because I'm waiting for lints)