Ground work for Wino Calendar. (#475)

Wino Calendar abstractions.

Wino.Messages/Client/Calendar/VisibleDateRangeChangedMessage.cs

@@ -0,0 +1,11 @@
+using Wino.Core.Domain.Models.Calendar;
+
+namespace Wino.Messaging.Client.Calendar
+{
+    /// <summary>
+    /// Raised when calendar's visible date range is updated.
+    /// Used to update the background of the visible date range in CalendarView.
+    /// </summary>
+    /// <param name="DateRange">New visible date range.</param>
+    public record VisibleDateRangeChangedMessage(DateRange DateRange);
+}