Add data models for the new call layouts

This is a start at implementing the call layouts from the new designs. I've added data types to model the contents of each possible layout, and begun implementing the business logic to produce these layouts in the call view model.

src/state/ViewModel.ts

@@ -1,5 +1,5 @@
 /*
-Copyright 2023 New Vector Ltd
+Copyright 2023-2024 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.
@@ -14,20 +14,19 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { Subject } from "rxjs";
+import { ObservableScope } from "./ObservableScope";
 
 /**
  * An MVVM view model.
  */
 export abstract class ViewModel {
-  protected readonly destroyed = new Subject<void>();
+  protected readonly scope = new ObservableScope();
 
   /**
    * Instructs the ViewModel to clean up its resources. If you forget to call
    * this, there may be memory leaks!
    */
   public destroy(): void {
-    this.destroyed.next();
-    this.destroyed.complete();
+    this.scope.end();
   }
 }