Bind ListBox.SelectedItems again.

Was removed accidentally.

samples/ControlCatalog/Pages/ListBoxPage.xaml

@@ -10,7 +10,13 @@
               HorizontalAlignment="Center"
               Spacing="16">
       <StackPanel Orientation="Vertical" Spacing="8">
-        <ListBox Items="{Binding Items}" SelectedItem="{Binding SelectedItem}" AutoScrollToSelectedItem="True"  SelectionMode="{Binding SelectionMode}" Width="250" Height="350"></ListBox>
+        <ListBox Items="{Binding Items}"
+                 SelectedItem="{Binding SelectedItem}"
+                 SelectedItems="{Binding SelectedItems}"
+                 AutoScrollToSelectedItem="True"
+                 SelectionMode="{Binding SelectionMode}"
+                 Width="250"
+                 Height="350"/>
 
         <Button Command="{Binding AddItemCommand}">Add</Button>