MVVM - Introduce the concept of disposables to track event listeners, sub vms and so on

[MidhunSureshR]

Builds on from #30398

Previously, SubscriptionViewModel had the following mechanism for adding/removing any event listeners that the vm needs:

• addDownstreamSubscription is called when the number of view subscriptions change from 0 to 1.
• removeDownstreamSubscription is called when the number of view subscriptions reduce to 0.

This means that the vm only listens to any relevant events (that it needs to keep its state correct) as long as there is at least one view subscribed to it. This is a problem because if the view unmounts and then mounts again, meaning that it goes through subscribe -> unsubscribe -> subscribe, the view model can present the wrong state because it wasn't listening to relevant events. Therefore, the lifetime of the event listeners should equal the lifetime of the view model itself.
Issue is illustrated here: https://github.com/element-hq/element-web/blob/c64104ffeea69da579ad601f9a4e0af8624e05a9/src/shared-components/ViewModelSubIssueDemo.test.tsx

This PR borrows the concept of Disposables from hydrogen:
It provides a way to track any resource that needs to be relinquished eventually. This will usually be sub view models or event listeners.
Whoever creates the view model has the responsibility to call dispose() method on it to indicate that it is no longer needed.

Tracking sub view-models

class FooViewModel extends BaseViewModel {
    private barViewModel: BarViewModel;
    constructor() {
        // barViewModel will be disposed whenever this vm is disposed
        this.barViewModel = this.disposables.track(new BarViewModel());
    }
}

Tracking event listeners

class FooViewModel extends BaseViewModel {
    constructor() {
        // This listener will be removed whenever this vm is disposed
        this.disposables.trackListener(props.someEmitter, "SOME_EVENT", () => { this.doSomething(); });
    }
   
   doSomething() {
      // ...
   }
}

[florianduros]

It seems that the dispose function is never called for the two vms (AudioPlayer & TextuamEvent). For AudioPlayer, you can call in the unmount of the MAudioBody