TreeView: Reload node support to reload only a specific part of the tree

[njannink]

Add support for reloading a specific node of the tree, eg for when a child is added/removed from a specific node to force reevaluation of the expand toggle visability.

[stsrki]

@njannink made some optimizations. Can you recheck the behavior on your side?

[njannink]

Will check will also see if this fixes #6159

[njannink]

@stsrki I don't really know what is the best approach to continue with this. Biggest issue is that the TreeView does support ObservableCollection but only after a node has been expanded so that is making the API a bit confusing. Ideally for HasChildNodes you use something like an IObservable that you subscribe, but that would introduce a dependency on System.Reactive.

We could introduce:

 interface ITreeNode
{
    IEnumerable<T> GetChildren();
    bool HasChildren();
    event EventHandler ChildrenChanged;
}

And then do some automatic binding and also trigger reevalution when the ChildrenChanged is triggered.

Thoughts are welcome