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/ViewModels/Items/SettingsNewVm.cs

@@ -6,12 +6,12 @@ namespace ModernKeePass.ViewModels
 {
     public class SettingsNewVm
     {
-        private ISettings _settings;
+        private ISettingsService _settings;
 
-        public SettingsNewVm() : this(new SettingsService())
+        public SettingsNewVm() : this(SettingsService.Instance)
         { }
 
-        public SettingsNewVm(ISettings settings)
+        public SettingsNewVm(ISettingsService settings)
         {
             _settings = settings;
         }