State action callbacks

[adospace]

This PR adds the ability to register state callback functions inside item templates.

For example:

public override VisualNode Render()
{
    return new ContentPage
    {
        new CollectionView()
            .ItemsSource(State.Persons, RenderItem)
    };
}

private VisualNode RenderItem(IndexedPersonWithAddress item)
{
    return new Border()
    .ScaleX(() =>...use state)
    ;
}