TextBox: update command states on selection changes

src/Avalonia.Controls/TextBox.cs

@@ -260,7 +260,11 @@ namespace Avalonia.Controls
             set
             {
                 value = CoerceCaretIndex(value);
-                SetAndRaise(SelectionStartProperty, ref _selectionStart, value);
+                var changed = SetAndRaise(SelectionStartProperty, ref _selectionStart, value);
+                if (changed)
+                {
+                    UpdateCommandStates();
+                }
                 if (SelectionStart == SelectionEnd)
                 {
                     CaretIndex = SelectionStart;
@@ -278,7 +282,11 @@ namespace Avalonia.Controls
             set
             {
                 value = CoerceCaretIndex(value);
-                SetAndRaise(SelectionEndProperty, ref _selectionEnd, value);
+                var changed = SetAndRaise(SelectionEndProperty, ref _selectionEnd, value);
+                if (changed)
+                {
+                    UpdateCommandStates();
+                }
                 if (SelectionStart == SelectionEnd)
                 {
                     CaretIndex = SelectionEnd;