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/BigGrid.tsx

@@ -19,11 +19,11 @@ import { RectReadOnly } from "react-use-measure";
 import { FC, memo, ReactNode } from "react";
 import { zip } from "lodash";
 
-import { TileDescriptor } from "./VideoGrid";
 import { Slot } from "./NewVideoGrid";
 import { Layout } from "./Layout";
 import { count, findLastIndex } from "../array-utils";
 import styles from "./BigGrid.module.css";
+import { TileDescriptor } from "../state/CallViewModel";
 
 /**
  * A 1×1 cell in a grid which belongs to a tile.