Ground work for Wino Calendar. (#475)

Wino Calendar abstractions.

Wino.Messages/Client/Calendar/CalendarEventAdded.cs

@@ -0,0 +1,9 @@
+using Wino.Core.Domain.Interfaces;
+
+namespace Wino.Messaging.Client.Calendar
+{
+    /// <summary>
+    /// Raised when event is added to database.
+    /// </summary>
+    public record CalendarEventAdded(ICalendarItem CalendarItem);
+}