Prefer TemplateBinding over Binding with RelativeSource (#3930)

* Prefer TemplateBinding over Binding with RelativeSource

* Updates Expander control template

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

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Co-authored-by: Kevin Bost <kitokeboo@gmail.com>

Author: corvinsz

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>
 
@@ -316,7 +316,7 @@
                       HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                       VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                   <Grid.LayoutTransform>
-                    <RotateTransform Angle="{Binding Path=ExpandDirection, RelativeSource={RelativeSource AncestorType=Expander}, Converter={x:Static convertersInternal:ExpanderRotateAngleConverter.Instance}, ConverterParameter=-1}" />
+                    <RotateTransform Angle="{TemplateBinding ExpandDirection, Converter={x:Static convertersInternal:ExpanderRotateAngleConverter.Instance}, ConverterParameter=-1}" />
                   </Grid.LayoutTransform>
 
                   <ContentPresenter Name="PART_Content"