Adding MenuItems to ContextMenu can cause them to half-function

[grokys]

If we assign a list that doesn't raise change notifications to ContextMenu.Items and then add MenuItems to that list, the items get displayed but they don't display correct hover behavior:

var b = this.FindControl<Border>("b");
var c = b.ContextMenu;
var items = new List<MenuItem>();
var item = new MenuItem { Header = "foo" };

c.Items = items;

// `items` doesn't raise a `CollectionChanged` event
items.Add(item);

Now, it's incorrect to do this, but the result is hard to debug. See if we can do better here either by throwing or by recovering the situation.

[grokys]

The problem here is the ItemsControl.ItemCount is only updated when a collection changed event is fired, so when it comes to select the first item, ItemCount == 0 meaning it thinks index 0 is invalid.

We should fix this in #3388 by moving to ItemsSourceView.