add ShowEmpty prop for Empty control

src/Net_40/HandyControl_Net_40/Themes/Theme.xaml

@@ -8232,6 +8232,7 @@
 		<Setter Property="Description" Value="{StaticResource DescriptionTextBlock4Empty}" />
 		<Setter Property="HorizontalAlignment" Value="Center" />
 		<Setter Property="VerticalAlignment" Value="Center" />
+		<Setter Property="Visibility" Value="{Binding Path=(hc:Empty.ShowEmpty),RelativeSource={RelativeSource Self},Converter={StaticResource Boolean2VisibilityConverter}}" />
 		<Setter Property="Template">
 			<Setter.Value>
 				<ControlTemplate TargetType="hc:Empty">