Vertically center content in controls with drop-down chevrons

This matches ComboBox

src/Avalonia.Themes.Default/Controls/SplitButton.xaml

@@ -61,6 +61,8 @@
     <Setter Property="BorderThickness" Value="{DynamicResource SplitButtonBorderThemeThickness}" />
     <Setter Property="HorizontalAlignment" Value="Left" />
     <Setter Property="VerticalAlignment" Value="Center" />
+    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
+    <Setter Property="VerticalContentAlignment" Value="Center" />
     <!--<Setter Property="UseSystemFocusVisuals" Value="True" />
     <Setter Property="FocusVisualMargin" Value="-3" />-->
     <Setter Property="KeyboardNavigation.IsTabStop" Value="True" />

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

@@ -23,6 +23,8 @@
     <Setter Property="Padding" Value="{DynamicResource ButtonPadding}" />
     <Setter Property="HorizontalAlignment" Value="Left" />
     <Setter Property="VerticalAlignment" Value="Center" />
+    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
+    <Setter Property="VerticalContentAlignment" Value="Center" />
     <!--<Setter Property="UseSystemFocusVisuals" Value="{StaticResource UseSystemFocusVisuals}" />
     <Setter Property="FocusVisualMargin" Value="-3" />-->
     <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />

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

@@ -37,6 +37,8 @@
     <Setter Property="BorderThickness" Value="{DynamicResource SplitButtonBorderThemeThickness}" />
     <Setter Property="HorizontalAlignment" Value="Left" />
     <Setter Property="VerticalAlignment" Value="Center" />
+    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
+    <Setter Property="VerticalContentAlignment" Value="Center" />
     <!--<Setter Property="UseSystemFocusVisuals" Value="True" />
     <Setter Property="FocusVisualMargin" Value="-3" />-->
     <Setter Property="KeyboardNavigation.IsTabStop" Value="True" />