Add chapter 4 completed solution

Chapter04/Complete/MyMediaCollection/Interfaces/IDataService.cs

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+using MyMediaCollection.Enums;
+using MyMediaCollection.Model;
+using System.Collections.Generic;
+
+namespace MyMediaCollection.Interfaces
+{
+    public interface IDataService
+    {
+        IList<MediaItem> GetItems();
+        MediaItem GetItem(int id);
+        int AddItem(MediaItem item);
+        void UpdateItem(MediaItem item);
+        IList<ItemType> GetItemTypes();
+        Medium GetMedium(string name);
+        IList<Medium> GetMediums();
+        IList<Medium> GetMediums(ItemType itemType);
+        IList<LocationType> GetLocationTypes();
+    }
+}

Chapter04/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();
+    }
+}