Fix alignment helper not working and missing icon for clear button.

src/Avalonia.Diagnostics/Diagnostics/Controls/FilterTextBox.axaml

@@ -16,6 +16,8 @@
         <StackPanel Orientation="Horizontal" Spacing="1">
           <Button Margin="0,0,2,0"
                   Classes="textBoxClearButton"
+                  Theme="{StaticResource SimpleTextBoxClearButtonTheme}"
+                  Focusable="False"
                   ToolTip.Tip="Clear"
                   Cursor="Hand"
                   Command="{Binding $parent[TextBox].Clear}"

src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlLayoutViewModel.cs

@@ -103,13 +103,13 @@ namespace Avalonia.Diagnostics.ViewModels
         public HorizontalAlignment HorizontalAlignment
         {
             get => _horizontalAlignment;
-            private set => RaiseAndSetIfChanged(ref _horizontalAlignment, value);
+            set => RaiseAndSetIfChanged(ref _horizontalAlignment, value);
         }
 
         public VerticalAlignment VerticalAlignment
         {
             get => _verticalAlignment;
-            private set => RaiseAndSetIfChanged(ref _verticalAlignment, value);
+            set => RaiseAndSetIfChanged(ref _verticalAlignment, value);
         }
 
         public bool HasPadding { get; }