fix bug introduced in dropdownitem the clears :selected psuedo class

from any control that has IsFocusedProperty.

src/Avalonia.Controls/DropDownItem.cs

@@ -22,14 +22,13 @@ namespace Avalonia.Controls
         static DropDownItem()
         {
             FocusableProperty.OverrideDefaultValue<DropDownItem>(true);
-            IsFocusedProperty.Changed.Subscribe(x =>
+        }
-            {
-                var sender = x.Sender as IControl;
 
-                if (sender != null)
+        public DropDownItem()
-                {
+        {
-                    ((IPseudoClasses)sender.Classes).Set(":selected", (bool)x.NewValue);
+            this.GetObservable(DropDownItem.IsFocusedProperty).Subscribe(focused =>
-                }
+            {
+                PseudoClasses.Set(":selected", focused);                
             });
         }