Add starter and complete projects for ch 7

Chapter07/Complete/MyMediaCollection/Interfaces/IDataService.cs

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+using MyMediaCollection.Enums;
+using MyMediaCollection.Model;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace MyMediaCollection.Interfaces
+{
+    public interface IDataService
+    {
+        Task<IList<MediaItem>> GetItemsAsync();
+        Task<MediaItem> GetItemAsync(int id);
+        Task<int> AddItemAsync(MediaItem item);
+        Task UpdateItemAsync(MediaItem item);
+        Task DeleteItemAsync(MediaItem item);
+        IList<ItemType> GetItemTypes();
+        Medium GetMedium(string name);
+        IList<Medium> GetMediums();
+        IList<Medium> GetMediums(ItemType itemType);
+        IList<LocationType> GetLocationTypes();
+        Task InitializeDataAsync();
+    }
+}

Chapter07/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();
+    }
+}

Chapter07/Complete/MyMediaCollection/Interfaces/IValidatable.cs

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+namespace MyMediaCollection.Interfaces
+{
+    public interface IValidatable
+    {
+        void Validate(string memberName, object value);
+    }
+}