message: fix broken sorting binding
diff: mah:DataGridHelper.EnableCellEditAssist="True"
  AutoGenerateColumns="False">
  <DataGrid.Columns>
  - <DataGridTemplateColumn x:Name="ColWarningColumn" Width="20" CanUserResize="False" CanUserReorder="False" SortMemberPath="IsHidden">
  + <DataGridTemplateColumn x:Name="ColWarningColumn" Width="20" CanUserResize="False" CanUserReorder="False" SortMemberPath="IsRequired">
  <DataGridTemplateColumn.CellTemplate>
  <DataTemplate>
  <controls:WarningWithTooltip Margin="4,4,0,0" Visibility="{Binding IsRequired, Converter={StaticResource BooleanToVisibilityConverter}}" />
  