Fire Page Appearing event

There's not really a great time to do this so I settled on
firing it when it's converted to a Ooui element.

Fixes #116

Ooui.Forms/PageExtensions.cs

@@ -25,6 +25,8 @@ namespace Xamarin.Forms
             if (existingRenderer != null)
                 return existingRenderer.NativeView;
 
+            ((IPageController)page).SendAppearing ();
+
             return CreateElement (page);
         }
 

Samples/DisplayAlertPage.xaml

@@ -22,6 +22,7 @@
             </StackLayout>
             <Button Text="Tap to Display Alert"
                     Clicked="OnButtonClicked" />
+            <Label x:Name="status" />
         </StackLayout>
     </ContentPage.Content>
 </ContentPage>

Samples/DisplayAlertPage.xaml.cs

@@ -14,6 +14,13 @@ namespace Samples
 			InitializeComponent ();
 		}
 
+        protected override void OnAppearing ()
+        {
+            base.OnAppearing ();
+
+            status.Text = "Page appeared";
+        }
+
         public async void OnButtonClicked(object sender, EventArgs args)
         {
             activity.IsRunning = true;