Fix issues with ImageEx Lazy Loading Sample

Can now use the sample button as a toggle.
Close button is now visible.
Made grid taller so sample is more apparent on high resolution monitors.

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/ImageEx/ImageExLazyLoadingControl.xaml

@@ -11,7 +11,7 @@
         <Grid Margin="40"
               Background="White">
             <ScrollViewer VerticalScrollBarVisibility="Auto">
-                <Grid Height="3000">
+                <Grid Height="6000">
                     <Border Width="200"
                             Height="200"
                             HorizontalAlignment="Center"
@@ -35,7 +35,8 @@
                     VerticalAlignment="Top"
                     Background="Transparent"
                     BorderThickness="0"
-                    Click="CloseButton_Click">
+                    Click="CloseButton_Click"
+                    Foreground="Black">
                 <SymbolIcon Symbol="Cancel" />
             </Button>
         </Grid>

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/ImageEx/ImageExPage.xaml.cs

@@ -82,7 +82,15 @@ namespace Microsoft.Toolkit.Uwp.SampleApp.SamplePages
 
                 if (lazyLoadingControlHost != null)
                 {
-                    lazyLoadingControlHost.Child = imageExLazyLoadingControl;
+                    // Allow this to act as a toggle.
+                    if (lazyLoadingControlHost.Child == null)
+                    {
+                        lazyLoadingControlHost.Child = imageExLazyLoadingControl;
+                    }
+                    else
+                    {
+                        lazyLoadingControlHost.Child = null;
+                    }
                 }
             });