Prototype: Eliminate TreeNodeVM

[Sewer56]

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Checks off Consider unifying TreeNodeVM with the underlying flat item view model. in
#1048

Successor to:

• Move IsExpanded to Child Items #1053

Need to merge previous PR if you want a slightly cleaner diff against main, as this PR builds upon it.
(Or just look at the new commits since that PR in isolation)

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This is a prototype which shows an implementation of inlining what was formerly TreeNodeVM directly into the ViewModels, for cleanliness. This PR adds the new system for doing so via IDynamicDataTreeItem<TItem, TKey>, and implements it for the View Mod Files view.

This PR is purely for feedback/iteration on the idea we had. i.e. a working prototype.
For @Al12rs and anyone else interested.

Notable Differences:

DynamicData tree creation changes from:

cache.Connect()
    .TransformToTree(model => model.ParentKey)
    .Transform(node => new ModFileNode(node))
    .Bind(out result)
    .Subscribe(); // force evaluation

To

cache.Connect()
    .TransformToTree(model => model.ParentKey)
    .Transform(node => node.Item.Initialize(node))
    .Bind(out result)
    .Subscribe(); // force evaluation

ViewModels that inherit from IDynamicDataTreeItem must expose 2 more properties.

// Example: IFileTreeNodeViewModel
public ReadOnlyObservableCollection<IFileTreeNodeViewModel>? Children { get; set; }
public IFileTreeNodeViewModel? Parent { get; set; }

Aside from that, no difference in usability from consumer end. The reactiveness works just a-ok, we can access children, access parents. etc.

The TreeNodeVM wrapper is no longer needed, the hack of re-broadcasting PropertyChanged from inner Item is no longer needed, and code that interacts with the ViewModel no longer needs to have .Item all over the place. This saves on the amount of code we have to write.

Logically speaking, there's no difference in how things operate behind the scenes. The difference is, Node<TObject,TKey> now holds a transitive reference to TItem instead of TreeNodeVM<TItem, TKey> via the .Children.Connect() subscription.