Make DropDown in ControlCatalog display correctly.

samples/ControlCatalog/Pages/CarouselPage.paml

@@ -20,9 +20,9 @@
       </Button>
     </StackPanel>
 
-    <StackPanel Orientation="Horizontal">
-      <TextBlock>Transition</TextBlock>
-      <DropDown SelectedIndex="0">
+    <StackPanel Orientation="Horizontal" Gap="4">
+      <TextBlock VerticalAlignment="Center">Transition</TextBlock>
+      <DropDown SelectedIndex="0" VerticalAlignment="Center">
         <ListBoxItem>None</ListBoxItem>
         <ListBoxItem>Slide</ListBoxItem>
         <ListBoxItem>Crossfade</ListBoxItem>

src/Perspex.Controls/DropDown.cs

@@ -83,6 +83,12 @@ namespace Perspex.Controls
             return new ItemContainerGenerator<ListBoxItem>(this, ListBoxItem.ContentProperty);
         }
 
+        protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
+        {
+            base.OnAttachedToLogicalTree(e);
+            this.UpdateSelectionBoxItem(this.SelectedItem);
+        }
+
         protected override void OnKeyDown(KeyEventArgs e)
         {
             base.OnKeyDown(e);
@@ -149,7 +155,19 @@ namespace Perspex.Controls
 
         private void SelectedItemChanged(PerspexPropertyChangedEventArgs e)
         {
-            var control = e.NewValue as IControl;
+            UpdateSelectionBoxItem(e.NewValue);
+        }
+
+        private void UpdateSelectionBoxItem(object item)
+        {
+            var contentControl = item as IContentControl;
+
+            if (contentControl != null)
+            {
+                item = contentControl.Content;
+            }
+
+            var control = item as IControl;
 
             if (control != null)
             {
@@ -169,7 +187,7 @@ namespace Perspex.Controls
             }
             else
             {
-                SelectionBoxItem = e.NewValue;
+                SelectionBoxItem = item;
             }
         }
     }

src/Perspex.Themes.Default/DropDown.paml

@@ -4,6 +4,7 @@
     <Setter Property="BorderThickness" Value="2"/>
     <Setter Property="HorizontalContentAlignment" Value="Center"/>
     <Setter Property="VerticalContentAlignment" Value="Center"/>
+    <Setter Property="Padding" Value="4"/>
     <Setter Property="Template">
       <ControlTemplate>
         <Border Background="{TemplateBinding Background}"