Updates Expander control template

Changes `ContentSite` border to be unnamed and binds `ExpandDirection` directly from the `TemplateBinding`.

Author: Keboo

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Expander.xaml

@@ -303,11 +303,11 @@
                             Opacity="0.87"
                             TextElement.FontSize="{TemplateBinding wpf:ExpanderAssist.HeaderFontSize}" />
 
-              <Border Name="ContentSite">
+              <Border>
                 <Border.LayoutTransform>
                   <TransformGroup>
                     <ScaleTransform x:Name="ContentSiteScaleTransform" />
-                    <RotateTransform Angle="{Binding Path=ExpandDirection, RelativeSource={RelativeSource AncestorType=Expander}, Converter={x:Static convertersInternal:ExpanderRotateAngleConverter.Instance}}" />
+                    <RotateTransform Angle="{TemplateBinding ExpandDirection, Converter={x:Static convertersInternal:ExpanderRotateAngleConverter.Instance}}" />
                   </TransformGroup>
                 </Border.LayoutTransform>