Add starter code for chapter 8

No modifications for Complete folder yet.

Chapter08/Complete/MyMediaCollection/Interfaces/INavigationService.cs

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+using System;
+
+namespace MyMediaCollection.Interfaces
+{
+    public interface INavigationService
+    {
+        string CurrentPage { get; }
+        void NavigateTo(string page);
+        void NavigateTo(string page, object parameter);
+        void GoBack();
+    }
+}