Ground work for Wino Calendar. (#475)

Wino Calendar abstractions.

Wino.Core.ViewModels/CalendarBaseViewModel.cs

@@ -0,0 +1,13 @@
+using CommunityToolkit.Mvvm.Messaging;
+using Wino.Core.Domain.Interfaces;
+using Wino.Messaging.Client.Calendar;
+
+namespace Wino.Core.ViewModels
+{
+    public class CalendarBaseViewModel : CoreBaseViewModel, IRecipient<CalendarEventAdded>
+    {
+        public void Receive(CalendarEventAdded message) => OnCalendarEventAdded(message.CalendarItem);
+
+        protected virtual void OnCalendarEventAdded(ICalendarItem calendarItem) { }
+    }
+}