added context menu to default theme

src/Avalonia.Themes.Default/TextBox.xaml

@@ -13,6 +13,7 @@
     <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}"/>
     <Setter Property="SelectionBrush" Value="{DynamicResource HighlightBrush}"/>
     <Setter Property="SelectionForegroundBrush" Value="{DynamicResource HighlightForegroundBrush}"/>
+    <Setter Property="ContextMenu" Value="{StaticResource DefaultTextBoxContextMenu}" />
     <Setter Property="Padding" Value="4"/>
     <Setter Property="ContextMenu" Value="{StaticResource DefaultTextBoxContextMenu}" />
     <Setter Property="Template">