Remove TSource default constructor requirement for IncrementalLoadingCollection

Addressing #616

Author: hermitdave

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

@@ -41,7 +41,8 @@ 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 collection = new IncrementalLoadingCollection<PeopleSource, Person>();
+            var source = new PeopleSource();
+            var collection = new IncrementalLoadingCollection<PeopleSource, Person>(source);
             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

@@ -117,7 +117,9 @@ 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 () =>
                 {

Microsoft.Toolkit.Uwp/IncrementalLoadingCollection/IncrementalLoadingCollection.cs

@@ -38,7 +38,7 @@ namespace Microsoft.Toolkit.Uwp
     /// <seealso cref="ISupportIncrementalLoading"/>
     public class IncrementalLoadingCollection<TSource, IType> : ObservableCollection<IType>,
          ISupportIncrementalLoading
-         where TSource : IIncrementalSource<IType>, new()
+         where TSource : IIncrementalSource<IType>
     {
         /// <summary>
         /// Gets a value indicating the source of incremental loading.
@@ -117,27 +117,6 @@ private set
             }
         }
 
-        /// <summary>
-        /// Initializes a new instance of the <see cref="IncrementalLoadingCollection{TSource, IType}"/> class optionally specifying how many items to load for each data page.
-        /// </summary>
-        /// <param name="itemsPerPage">
-        /// The number of items to retrieve for each call. Default is 20.
-        /// </param>
-        /// <param name="onStartLoading">
-        /// An <see cref="Action"/> that is called when a retrieval operation begins.
-        /// </param>
-        /// <param name="onEndLoading">
-        /// An <see cref="Action"/> that is called when a retrieval operation ends.
-        /// </param>
-        /// <param name="onError">
-        /// An <see cref="Action"/> that is called if an error occours during data retrieval.
-        /// </param>
-        /// <seealso cref="IIncrementalSource{TSource}"/>
-        public IncrementalLoadingCollection(int itemsPerPage = 20, Action onStartLoading = null, Action onEndLoading = null, Action<Exception> onError = null)
-            : this(new TSource(), itemsPerPage, onStartLoading, onEndLoading, onError)
-        {
-        }
-
         /// <summary>
         /// Initializes a new instance of the <see cref="IncrementalLoadingCollection{TSource, IType}"/> class using the specified <see cref="IIncrementalSource{TSource}"/> implementation and, optionally, how many items to load for each data page.
         /// </summary>

docs/helpers/IncrementalLoadingCollection.md

@@ -54,6 +54,7 @@ The *GetPagedItemsAsync* method is invoked everytime the view need to show more
 `IncrementalLoadingCollection` can then be bound to a [ListView](https://msdn.microsoft.com/library/windows/apps/windows.ui.xaml.controls.listview.aspx) or a [GridView-like](https://msdn.microsoft.com/library/windows/apps/windows.ui.xaml.controls.gridview.aspx) control:
 
 ```csharp
+    var source = new PeopleSource();
     var collection = new IncrementalLoadingCollection<PeopleSource, Person>();
     PeopleListView.ItemsSource = collection;
 ```
@@ -62,7 +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. If the source isn't provided to the constructor, it is created automatically. | IIncrementalSource |  
+| source | An implementation of the **IIncrementalSource** interface that contains the logic to actually load data incrementally. | 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) |