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/Aop/DatabaseChangedProxy.cs

@@ -0,0 +1,26 @@
+using System.Reflection;
+using ModernKeePass.Interfaces;
+
+namespace ModernKeePass.Aop
+{
+    public class DatabaseChangedProxy<T>: IProxyInvocationHandler
+    {
+        private readonly T _decorated;
+        private readonly IDatabaseService _databaseService;
+
+        public DatabaseChangedProxy(T decorated, IDatabaseService databaseService)
+        {
+            _decorated = decorated;
+            _databaseService = databaseService;
+        }
+
+        public object Invoke(object proxy, MethodInfo method, object[] parameters)
+        {
+            object retVal = null;
+            retVal = method.Invoke(proxy, parameters);
+            _databaseService.HasChanged = true;
+
+            return retVal;
+        }
+    }
+}