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/room/useFullscreen.ts

@@ -18,9 +18,9 @@ limitations under the License.
 import { logger } from "matrix-js-sdk/src/logger";
 import { useCallback, useLayoutEffect, useRef } from "react";
 
-import { TileDescriptor } from "../video-grid/VideoGrid";
 import { useReactiveState } from "../useReactiveState";
 import { useEventTarget } from "../useEvents";
+import { TileDescriptor } from "../state/CallViewModel";
 
 const isFullscreen = (): boolean =>
   Boolean(document.fullscreenElement) ||