Fix bug in PerspexReadOnlyList.

The source collection is not guaranteed to implement IList so do an
explicit .ToList() on it.

Perspex.Base/Collections/PerspexReadOnlyListView.cs

@@ -75,7 +75,7 @@ namespace Perspex.Collections
                     {
                         var ev = new NotifyCollectionChangedEventArgs(
                             NotifyCollectionChangedAction.Add,
-                            this.source,
+                            this.source.ToList(),
                             0);
                         this.CollectionChanged(this, ev);
                     }