Added subheaders for Pickers.xaml of demo

Author: wongjiahau

MainDemo.Wpf/Pickers.xaml

@@ -10,8 +10,8 @@
              d:DesignHeight="300" d:DesignWidth="300">
     <AdornerDecorator>
         <ScrollViewer>
-            <codeDisplayer:XamlDisplayerPanel >
-                <TextBlock >Classic WPF DatePicker control with Material Design theme, and new TimePicker control:</TextBlock>
+            <codeDisplayer:XamlDisplayerPanel HorizontalAlignment="Center">
+                <TextBlock Style="{StaticResource MaterialDesignSubheadingTextBlock}" codeDisplayer:XamlDisplayer.DisplayCode="False">Classic WPF DatePicker control with Material Design theme, and new TimePicker control:</TextBlock>
                 <DatePicker Width="100" HorizontalAlignment="Left" Margin="0 16 0 0" materialDesign:HintAssist.Hint="Pick Date" Style="{StaticResource MaterialDesignFloatingHintDatePicker}" />
                 <DatePicker x:Name="FutureDatePicker" Width="100" HorizontalAlignment="Left" Margin="0 16 0 0" 
                         materialDesign:HintAssist.Hint="Future Date">
@@ -53,7 +53,7 @@
                     </materialDesign:TimePicker.Text>
                 </materialDesign:TimePicker>
 
-                <TextBlock MaxWidth="200" TextWrapping="Wrap">By combining the DialogHost (see MainWindow.xaml) and the Calendar and Clock controls, custom popups can be built.</TextBlock>
+                <TextBlock Style="{StaticResource MaterialDesignSubheadingTextBlock}" codeDisplayer:XamlDisplayer.DisplayCode="False">By combining the DialogHost (see MainWindow.xaml) and the Calendar and Clock controls, custom popups can be built.</TextBlock>
                 <StackPanel  Orientation="Horizontal" Margin="0 32 0 0">
                     <TextBlock Text="{Binding Date, StringFormat=d}" VerticalAlignment="Center" FontSize="24" />
                     <Button Margin="8 0 0 0" Content="..."