Make Separator look correct.

Previously there was a `MenuItem > Separator` selector, but I don't really see any reason to just apply this style directly to `Separator` itself. It's mainly used in menus and if one wants to use it elsewhere then they'd need to set these properties anyway.

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

@@ -26,6 +26,7 @@
           <MenuItem Header="Child 1"/>
           <MenuItem Header="Child 2"/>
           <MenuItem Header="Child 3"/>
+          <Separator/>
           <MenuItem Header="Child 4"/>
           <MenuItem Header="Child 5"/>
           <MenuItem Header="Child 6"/>
@@ -204,9 +205,7 @@
     <Style Selector="^:separator">
       <Setter Property="Template">
         <ControlTemplate>
-          <Separator Background="{DynamicResource SystemControlForegroundBaseMediumLowBrush}"
-                     Margin="{DynamicResource MenuFlyoutSeparatorThemePadding}"
-                     Height="{DynamicResource MenuFlyoutSeparatorThemeHeight}"/>
+          <Separator/>
         </ControlTemplate>
       </Setter>
     </Style>

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

@@ -1,7 +1,11 @@
 <ResourceDictionary xmlns="https://github.com/avaloniaui"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <ControlTheme x:Key="{x:Type Separator}" TargetType="Separator">
+    <Setter Property="Background" Value="{DynamicResource SystemControlForegroundBaseMediumLowBrush}" />
     <Setter Property="Focusable" Value="False" />
+    <Setter Property="Height" Value="{DynamicResource MenuFlyoutSeparatorThemeHeight}" />
+    <Setter Property="HorizontalAlignment" Value="Stretch" />
+    <Setter Property="Margin" Value="{DynamicResource MenuFlyoutSeparatorThemePadding}" />
     <Setter Property="Template">
       <ControlTemplate>
         <Border Padding="{TemplateBinding Margin}"