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/IDatabaseService.cs

@@ -0,0 +1,34 @@
+using System.Threading.Tasks;
+using Windows.Storage;
+using ModernKeePass.ViewModels;
+using ModernKeePassLib;
+using ModernKeePassLib.Cryptography.KeyDerivation;
+using ModernKeePassLib.Keys;
+
+namespace ModernKeePass.Interfaces
+{
+    public interface IDatabaseService
+    {
+        string Name { get; }
+        bool RecycleBinEnabled { get; set; }
+        //int Status { get; set; }
+        GroupVm RootGroup { get; set; }
+        GroupVm RecycleBin { get; set; }
+        StorageFile DatabaseFile { get; set; }
+        PwUuid DataCipher { get; set; }
+        PwCompressionAlgorithm CompressionAlgorithm { get; set; }
+        KdfParameters KeyDerivation { get; set; }
+        bool IsOpen { get; }
+        bool IsFileOpen { get; }
+        bool IsClosed { get; }
+        bool HasChanged { get; set; }
+
+        Task Open(CompositeKey key, bool createNew);
+        void UpdateCompositeKey(CompositeKey key);
+        void Save();
+        void Save(StorageFile file);
+        void CreateRecycleBin();
+        void AddDeletedItem(PwUuid id);
+        Task Close();
+    }
+}