Re-enabled write mode in KeePassLib

Set database as part of App (so, Singleton)
Changed main page view model
WIP and testing saving mechanism

ModernKeePass/Pages/GroupDetailPage.xaml.cs

@@ -89,5 +89,11 @@ namespace ModernKeePass.Pages
             var listView = sender as ListView;
             Frame.Navigate(typeof(EntryDetailPage), listView.SelectedItem as EntryVm);
         }
+
+        private void addGroupButton_Click(object sender, Windows.UI.Xaml.RoutedEventArgs e)
+        {
+            var group = DataContext as GroupVm;
+            group.AddGroup("New group");
+        }
     }
 }