Start refactoring some business logic into view models

As Element Call grows in complexity, it has become a pain point that our business logic remains so tightly coupled to the UI code. In particular, this has made testing difficult, and the complex semantics of React hooks are not a great match for arbitrary business logic. Here, I show the beginnings of what it would look like for us to adopt the MVVM pattern. I've created a CallViewModel and TileViewModel that expose their state to the UI as rxjs Observables, as well as a couple of helper functions for consuming view models in React code.

This should contain no user-visible changes, but we need to watch out for regressions particularly around focus switching and promotion of speakers, because this was the logic I chose to refactor first.

src/video-grid/NewVideoGrid.tsx

@@ -32,7 +32,6 @@ import styles from "./NewVideoGrid.module.css";
 import {
   VideoGridProps as Props,
   TileSpring,
-  TileDescriptor,
   ChildrenProperties,
 } from "./VideoGrid";
 import { useReactiveState } from "../useReactiveState";
@@ -40,6 +39,7 @@ import { useMergedRefs } from "../useMergedRefs";
 import { TileWrapper } from "./TileWrapper";
 import { BigGrid } from "./BigGrid";
 import { useLayout } from "./Layout";
+import { TileDescriptor } from "../state/CallViewModel";
 
 interface Rect {
   x: number;