Move DataGrid theme to Themes directory.

samples/ControlCatalog/App.xaml

@@ -4,7 +4,7 @@
   <Application.Styles>
       <StyleInclude Source="avares://Avalonia.Themes.Default/DefaultTheme.xaml"/>
       <StyleInclude Source="avares://Avalonia.Themes.Default/Accents/BaseLight.xaml"/>
-      <StyleInclude Source="resm:Avalonia.Controls.DataGrid.Default.xaml?assembly=Avalonia.Controls.DataGrid"/>
+      <StyleInclude Source="resm:Avalonia.Controls.DataGrid.Themes.Default.xaml?assembly=Avalonia.Controls.DataGrid"/>
     <Style Selector="TextBlock.h1">
       <Setter Property="FontSize" Value="{DynamicResource FontSizeLarge}"/>
       <Setter Property="FontWeight" Value="Medium"/>