App: Make HintContextChange() synchronous by using MainDispatcher

Elwig/Windows/MailWindow.xaml.cs

@@ -515,7 +515,7 @@ namespace Elwig.Windows {
                         var b = new Billing(year);
                         await b.FinishSeason();
                         await b.CalculateBuckets();
-                        await App.HintContextChange();
+                        App.HintContextChange();
 
                         dcData[year] = await DeliveryConfirmationDeliveryData.ForSeason(ctx.DeliveryParts, year);
                     } catch (Exception exc) {