[#43] App: Use FileSystemWatcher to renew contexts on demand

Elwig/Windows/PaymentVariantsWindow.xaml.cs

@@ -36,8 +36,7 @@ namespace Elwig.Windows {
             }
         }
 
-        protected override async Task OnRenewContext() {
-            using var ctx = new AppDbContext();
+        protected override async Task OnRenewContext(AppDbContext ctx) {
             ControlUtils.RenewItemsSource(PaymentVariantList, await ctx.PaymentVariants
                 .Where(v => v.Year == Year)
                 .OrderBy(v => v.AvNr)