Convenience extension methods for adding extra behavior before/after an existing mapping.

Adds two extensions:
propertyMapper.AppendToMapping(string key, Action<TViewHandler, TVirtualView> method)
propertyMapper.PrependToMapping(string key, Action<TViewHandler, TVirtualView> method)

Given a mapper like

var mapper = new PropertyMapper<IView, IViewHandler>
{
	[nameof(IView.Background)] = (r, v) => Debug.WriteLine("Original behavior")
};

We can do this

mapper.AppendToMapping(nameof(IView.Background), (h, v) => Debug.WriteLine("Additional behavior"));

And the output when the Background property updates would be

Original behavior
Additional behavior

Prepend works the other way around - the new behavior runs first.