Changed most services to singletons

Refactor the Database Service (no more enum, ...)
Restored the Donate page with Paypal web page
Added (but not working) MS App Center integration
Corrected tests accordingly
WIP AOP to detect database changes

ModernKeePass/Interfaces/IRecentService.cs

@@ -0,0 +1,15 @@
+using System.Collections.ObjectModel;
+using System.Threading.Tasks;
+using Windows.Storage;
+
+namespace ModernKeePass.Interfaces
+{
+    public interface IRecentService
+    {
+        int EntryCount { get; }
+        Task<IStorageItem> GetFileAsync(string token);
+        ObservableCollection<IRecentItem> GetAllFiles(bool removeIfNonExistant = true);
+        void Add(IStorageItem file, string metadata);
+        void ClearAll();
+    }
+}