Implementing Autocompletebox

[StanleyGoldman]

This pull request is the set of changes made to implement AutoCompleteBox

[jcansdale]

I just got the following exception/crash after @-ing myself and clicking on the avatar.

Application: devenv.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.InvalidOperationException
   at GitHub.UI.Controls.AutoCompleteBox.AutoCompleteTextInputExtensions.GetExpandedText(GitHub.UI.IAutoCompleteTextInput, System.String, Int32)
   at GitHub.UI.AutoCompleteBox.ExpandSuggestion(System.String)
   at GitHub.UI.AutoCompleteBox.OnAdapterSelectionComplete(System.Object, System.Windows.RoutedEventArgs)
   at GitHub.UI.SelectorSelectionAdapter.OnCommit(System.Object, System.Windows.RoutedEventArgs)
   at GitHub.UI.SelectorSelectionAdapter.OnCommit()
   at GitHub.UI.SelectorSelectionAdapter.OnSelectorMouseLeftButtonUp(System.Object, System.Windows.Input.MouseButtonEventArgs)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(System.Delegate, System.Object)
   at System.Windows.RoutedEventArgs.InvokeHandler(System.Delegate, System.Object)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(System.Object, System.Windows.RoutedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(System.Object, System.Windows.RoutedEventArgs, Boolean)
   at System.Windows.UIElement.ReRaiseEventAs(System.Windows.DependencyObject, System.Windows.RoutedEventArgs, System.Windows.RoutedEvent)
   at System.Windows.UIElement.OnMouseUpThunk(System.Object, System.Windows.Input.MouseButtonEventArgs)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(System.Delegate, System.Object)
   at System.Windows.RoutedEventArgs.InvokeHandler(System.Delegate, System.Object)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(System.Object, System.Windows.RoutedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(System.Object, System.Windows.RoutedEventArgs, Boolean)
   at System.Windows.UIElement.RaiseEventImpl(System.Windows.DependencyObject, System.Windows.RoutedEventArgs)
   at System.Windows.ContentElement.RaiseTrustedEvent(System.Windows.RoutedEventArgs)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(System.Windows.Input.InputEventArgs)
   at System.Windows.Input.InputProviderSite.ReportInput(System.Windows.Input.InputReport)
   at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr, System.Windows.Input.InputMode, Int32, System.Windows.Input.RawMouseActions, Int32, Int32, Int32)
   at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr, MS.Internal.Interop.WindowMessage, IntPtr, IntPtr, Boolean ByRef)
   at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
   at MS.Win32.HwndWrapper.WndProc(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(System.Object)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority, System.TimeSpan, System.Delegate, System.Object, Int32)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr, Int32, IntPtr, IntPtr)