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/Services/SingletonServiceBase.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace ModernKeePass.Services
+{
+    public abstract class SingletonServiceBase<T> where T : new()
+    {
+        private static readonly Lazy<T> LazyInstance =
+            new Lazy<T>(() => new T());
+
+        public static T Instance => LazyInstance.Value;
+    }
+}