Revert docs and usage to original constructor

Author: hermitdave

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/Incremental Loading Collection/IncrementalLoadingCollectionPage.xaml.cs

@@ -41,8 +41,7 @@ protected override void OnNavigatedTo(NavigationEventArgs e)
             base.OnNavigatedTo(e);
 
             // IncrementalLoadingCollection can be bound to a GridView or a ListView. In this case it is a ListView called PeopleListView.
-            var source = new PeopleSource();
-            var collection = new IncrementalLoadingCollection<PeopleSource, Person>(source);
+            var collection = new IncrementalLoadingCollection<PeopleSource, Person>();
             PeopleListView.ItemsSource = collection;
 
             // Binds the collection to the page DataContext in order to use its IsLoading and HasMoreItems properties.

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/Microsoft Graph Service/MicrosoftGraphPage.xaml.cs

@@ -117,9 +117,7 @@ private async void GetMessagesButton_Click(object sender, RoutedEventArgs e)
                 top = Convert.ToInt32(txtTop);
             }
 
-            var source = new MicrosoftGraphSource();
             var collection = new IncrementalLoadingCollection<MicrosoftGraphSource, Message>(
-                source,
                 top,
                 async () =>
                 {

docs/helpers/IncrementalLoadingCollection.md

@@ -63,7 +63,7 @@ The **IncrementalLoadingCollection** constructor accepts the following arguments
 
 | Name | Description | Type |
 | --- | --- | --- |
-| source | An implementation of the **IIncrementalSource** interface that contains the logic to actually load data incrementally. | IIncrementalSource |  
+| source | An implementation of the **IIncrementalSource** interface that contains the logic to actually load data incrementally. If the source isn't provided to the constructor, it is created automatically. | IIncrementalSource |  
 | itemsPerPage | The number of items to retrieve for each call. Default is 20. | [Integer](https://msdn.microsoft.com/library/windows/apps/System.Int32) |  
 | onStartLoading | (optional) An Action that is called when a retrieval operation begins. | [Action](https://msdn.microsoft.com/library/system.action.aspx) |  
 | onEndLoading | (optional) An Action that is called when a retrieval operation ends. | [Action](https://msdn.microsoft.com/library/system.action.aspx) |