Improve ButtonSpinner OnPointerWheel

- Only handle the PointerWheelEvent if the control has the focus
- When the pointer wheel should change the selection, mark the event as handled. So any surrounding ScrollViewer does not do the scroll.
- Make the Control Focusable by default in Style

src/Avalonia.Controls/ButtonSpinner.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 using Avalonia.Data;
@@ -196,13 +196,14 @@ namespace Avalonia.Controls
         protected override void OnPointerWheelChanged(PointerWheelEventArgs e)
         {
             base.OnPointerWheelChanged(e);
-            if (!e.Handled && AllowSpin)
+
+            if (AllowSpin && IsFocused)
             {
                 if (e.Delta.Y != 0)
                 {
                     var spinnerEventArgs = new SpinEventArgs(SpinEvent, (e.Delta.Y < 0) ? SpinDirection.Decrease : SpinDirection.Increase, true);
                     OnSpin(spinnerEventArgs);
-                    e.Handled = spinnerEventArgs.Handled;
+                    e.Handled = true;
                 }
             }
         }

src/Avalonia.Themes.Default/ButtonSpinner.xaml

@@ -5,6 +5,7 @@
     <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}"/>
     <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
     <Setter Property="VerticalContentAlignment" Value="Center"/>
+    <Setter Property="Focusable" Value="True"/>
   </Style>
   <Style Selector="ButtonSpinner /template/ RepeatButton">
     <Setter Property="Background" Value="Transparent"/>

src/Avalonia.Themes.Fluent/Controls/ButtonSpinner.xaml

@@ -64,6 +64,7 @@
     <Setter Property="FontSize" Value="{DynamicResource ControlContentThemeFontSize}" />
     <Setter Property="HorizontalContentAlignment" Value="Stretch" />
     <Setter Property="VerticalContentAlignment" Value="Center" />
+    <Setter Property="Focusable" Value="True"/>
     <Setter Property="Template">
       <ControlTemplate>
         <DataValidationErrors>