Ground work for Wino Calendar. (#475)

Wino Calendar abstractions.

Wino.Messages/Client/Calendar/CalendarSettingsUpdatedMessage.cs

@@ -0,0 +1,7 @@
+namespace Wino.Messaging.Client.Calendar
+{
+    /// <summary>
+    /// Raised when calendar settings are updated.
+    /// </summary>
+    public record CalendarSettingsUpdatedMessage;
+}