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

@@ -7,7 +7,7 @@ using ModernKeePassLib.Keys;
 
 namespace ModernKeePass.Interfaces
 {
-    public interface IDatabase
+    public interface IDatabaseService
     {
         string Name { get; }
         bool RecycleBinEnabled { get; set; }
@@ -21,6 +21,7 @@ namespace ModernKeePass.Interfaces
         bool IsOpen { get; }
         bool IsFileOpen { get; }
         bool IsClosed { get; }
+        bool HasChanged { get; set; }
 
         Task Open(CompositeKey key, bool createNew);
         void UpdateCompositeKey(CompositeKey key);

ModernKeePass/Interfaces/IProxyInvocationHandler.cs

@@ -0,0 +1,9 @@
+using System.Reflection;
+
+namespace ModernKeePass.Interfaces
+{
+    public interface IProxyInvocationHandler
+    {
+        object Invoke(object proxy, MethodInfo method, object[] parameters);
+    }
+}

ModernKeePass/Interfaces/IRecentService.cs

@@ -4,7 +4,7 @@ using Windows.Storage;
 
 namespace ModernKeePass.Interfaces
 {
-    public interface IRecent
+    public interface IRecentService
     {
         int EntryCount { get; }
         Task<IStorageItem> GetFileAsync(string token);

ModernKeePass/Interfaces/IResourceService.cs

@@ -1,6 +1,6 @@
 namespace ModernKeePass.Interfaces
 {
-    public interface IResource
+    public interface IResourceService
     {
         string GetResourceValue(string key);
     }

ModernKeePass/Interfaces/ISettingsService.cs

@@ -1,6 +1,6 @@
 namespace ModernKeePass.Interfaces
 {
-    public interface ISettings
+    public interface ISettingsService
     {
         T GetSetting<T>(string property);
         void PutSetting<T>(string property, T value);