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/state/useObservable.ts

@@ -0,0 +1,31 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { useEffect, useRef } from "react";
+import { BehaviorSubject, Observable } from "rxjs";
+
+/**
+ * React hook that creates an Observable from a changing value. The Observable
+ * replays its current value upon subscription, emits whenever the value
+ * changes, and completes when the component is unmounted.
+ */
+export function useObservable<T>(value: T): Observable<T> {
+  const subject = useRef<BehaviorSubject<T>>();
+  subject.current ??= new BehaviorSubject(value);
+  if (value !== subject.current.value) subject.current.next(value);
+  useEffect(() => subject.current!.complete(), []);
+  return subject.current;
+}