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/VideoGrid.tsx

@@ -44,6 +44,7 @@ import styles from "./VideoGrid.module.css";
 import { Layout } from "../room/LayoutToggle";
 import { TileWrapper } from "./TileWrapper";
 import { LayoutStatesMap } from "./Layout";
+import { TileDescriptor } from "../state/CallViewModel";
 
 interface TilePosition {
   x: number;
@@ -838,20 +839,6 @@ export interface VideoGridProps<T> {
   children: (props: ChildrenProperties<T>) => ReactNode;
 }
 
-// Represents something that should get a tile on the layout,
-// ie. a user's video feed or a screen share feed.
-export interface TileDescriptor<T> {
-  id: string;
-  focused: boolean;
-  isPresenter: boolean;
-  isSpeaker: boolean;
-  hasVideo: boolean;
-  local: boolean;
-  largeBaseSize: boolean;
-  placeNear?: string;
-  data: T;
-}
-
 export function VideoGrid<T>({
   items,
   layout,