Remove TextBox hacks.

Now that we can have attached properties in bindings.

src/Perspex.Controls/TextBox.cs

@@ -26,16 +26,6 @@ namespace Perspex.Controls
         public static readonly StyledProperty<bool> AcceptsTabProperty =
             PerspexProperty.Register<TextBox, bool>("AcceptsTab");
 
-        // TODO: Remove these when "(attached.property)" syntax is supported in bindings. They are
-        // bound to in the TextBox template, but ideally they should just be bound to the attached
-        // property.
-        public static readonly StyledProperty<bool> CanScrollHorizontally = 
-            ScrollViewer.CanScrollHorizontallyProperty.AddOwner<TextBox>();
-        public static readonly StyledProperty<ScrollBarVisibility> HorizontalScrollBarVisibilityProperty = 
-            ScrollViewer.HorizontalScrollBarVisibilityProperty.AddOwner<TextBox>();
-        public static readonly StyledProperty<ScrollBarVisibility> VerticalScrollBarVisibilityProperty = 
-            ScrollViewer.VerticalScrollBarVisibilityProperty.AddOwner<TextBox>();
-
         // TODO: Should CaretIndex, SelectionStart/End and Text be direct properties?
         public static readonly StyledProperty<int> CaretIndexProperty =
             PerspexProperty.Register<TextBox, int>("CaretIndex", validate: ValidateCaretIndex);

src/Perspex.Themes.Default/TextBox.xaml

@@ -28,9 +28,9 @@
               </TextBlock.IsVisible>
             </TextBlock>
             
-            <ScrollViewer CanScrollHorizontally="{TemplateBinding CanScrollHorizontally}"
-                          HorizontalScrollBarVisibility="{TemplateBinding HorizontalScrollBarVisibility}"
-                          VerticalScrollBarVisibility="{TemplateBinding VerticalScrollBarVisibility}">
+            <ScrollViewer CanScrollHorizontally="{TemplateBinding (ScrollViewer.CanScrollHorizontally)}"
+                          HorizontalScrollBarVisibility="{TemplateBinding (ScrollViewer.HorizontalScrollBarVisibility)}"
+                          VerticalScrollBarVisibility="{TemplateBinding (ScrollViewer.VerticalScrollBarVisibility)}">
               <Panel>
                 <TextBlock Name="watermark"
                            Opacity="0.5"