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.

test/video-grid/VideoGrid-test.ts

@@ -14,11 +14,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import {
-  Tile,
-  TileDescriptor,
-  reorderTiles,
-} from "../../src/video-grid/VideoGrid";
+import { TileDescriptor } from "../../src/state/CallViewModel";
+import { Tile, reorderTiles } from "../../src/video-grid/VideoGrid";
 
 const alice: Tile<unknown> = {
   key: "alice",