Fix IPerspexList.Count.

src/Perspex.Base/Collections/IPerspexList.cs

@@ -11,6 +11,11 @@ namespace Perspex.Collections
     /// <typeparam name="T">The type of the items in the list.</typeparam>
     public interface IPerspexList<T> : IList<T>, IPerspexReadOnlyList<T>
     {
+        /// <summary>
+        /// Gets the number of items in the list.
+        /// </summary>
+        new int Count { get; }
+
         /// <summary>
         /// Adds multiple items to the collection.
         /// </summary>