Fixes #3875. Add left and right tab sides with alignments to the TabView.

Terminal.Gui/Views/TabView/Tab.cs

@@ -22,11 +22,17 @@ public string DisplayText
         set
         {
             _displayText = value;
+            DisplayTextChanged?.Invoke (this, EventArgs.Empty);
             SetNeedsLayout ();
         }
     }
 
     /// <summary>The control to display when the tab is selected.</summary>
     /// <value></value>
     public View? View { get; set; }
+
+    /// <summary>
+    /// Raised when <see cref="DisplayText"/> changed.
+    /// </summary>
+    public event EventHandler? DisplayTextChanged;
 }