Add a xaml behavior and converter to support [None] items on top of lists

For lists where we want to support having no selection, it's handy to
have an "empty" item on top of the list for the user to select. The
behavior added here will add such an item to the top of a list if the
selected property has a value set in it. If the selected property is
null, the behavior removes the "empty" item.

The behavior requires that the `ItemsSource` binding of the ItemsControl
it's attached to be a MultiBinding, where the first entry is the bound
property that contains the "empty" item to show.

The `StickieListItemConverter` makes sure that the ItemsSource gets set
with the second entry of the MultiBinding.

Author: shana

src/GitHub.UI/Behaviours/AddEmptyItemToList.cs

@@ -0,0 +1,73 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Data;
+using System.Windows.Interactivity;
+
+namespace GitHub.UI
+{
+    public class AddEmptyItemToList : Behavior<ListBox>
+    {
+        object previousSelection = null;
+        dynamic defaultValue;
+        protected override void OnAttached()
+        {
+            base.OnAttached();
+
+            AssociatedObject.SelectionChanged += (s, e) =>
+            {
+                if (e.AddedItems.Count > 0)
+                {
+                    if (previousSelection == null)
+                    {
+                        previousSelection = AssociatedObject.SelectedItem;
+                        if (defaultValue == null)
+                        {
+                            var binding = AssociatedObject.GetBindingExpression(ListBox.ItemsSourceProperty);
+                            if (binding == null)
+                            {
+                                var m = BindingOperations.GetMultiBindingExpression(AssociatedObject, ListBox.ItemsSourceProperty);
+                                binding = (BindingExpression)m.BindingExpressions[0];
+                            }
+                            defaultValue = PropertyPathHelper.GetValue(binding.ResolvedSource, binding.ResolvedSourcePropertyName);
+                        }
+                        dynamic items = AssociatedObject.ItemsSource;
+                        items.Insert(0, defaultValue);
+                    }
+                    else if (AssociatedObject.SelectedIndex == 0)
+                    {
+                        dynamic items = AssociatedObject.ItemsSource;
+                        items.RemoveAt(0);
+                    }
+                }
+                if (e.RemovedItems.Count > 0)
+                {
+                    if (e.RemovedItems[0] == defaultValue)
+                    {
+                        previousSelection = null;
+                        AssociatedObject.SelectedItem = null;
+                    }
+                }
+            };
+        }
+
+        static class PropertyPathHelper
+        {
+            public static object GetValue(object obj, string propertyPath)
+            {
+                var binding = new Binding(propertyPath);
+                binding.Mode = BindingMode.OneTime;
+                binding.Source = obj;
+                BindingOperations.SetBinding(dummy, Dummy.ValueProperty, binding);
+                return dummy.GetValue(Dummy.ValueProperty);
+            }
+
+            static readonly Dummy dummy = new Dummy();
+
+            class Dummy : DependencyObject
+            {
+                public static readonly DependencyProperty ValueProperty =
+                    DependencyProperty.Register("Value", typeof(object), typeof(Dummy), new UIPropertyMetadata(null));
+            }
+        }
+    }
+}

src/GitHub.UI/Converters/StickieListItemConverter.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Globalization;
+
+namespace GitHub.UI
+{
+    public class StickieListItemConverter : MultiValueConverterMarkupExtension<StickieListItemConverter>
+    {
+        public override object Convert(object[] value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return value[1];
+        }
+    }
+}

src/GitHub.UI/GitHub.UI.csproj

@@ -77,6 +77,7 @@
     <Reference Include="WindowsBase" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="Behaviours\AddEmptyItemToList.cs" />
     <Compile Include="Behaviours\ClosePopupAction.cs" />
     <Compile Include="Behaviours\OpenPopupAction.cs" />
     <Compile Include="Controls\DropDownActionLinkBehaviour.cs" />
@@ -86,6 +87,7 @@
       <DependentUpon>OcticonPaths.resx</DependentUpon>
     </Compile>
     <Compile Include="Converters\DefaultValueConverter.cs" />
+    <Compile Include="Converters\StickieListItemConverter.cs" />
     <Compile Include="Resources.Designer.cs">
       <AutoGen>True</AutoGen>
       <DesignTime>True</DesignTime>