Make MenuItem.Icon a logical child of MenuItem.

So that it can be styled.

src/Perspex.Controls/MenuItem.cs

@@ -9,6 +9,7 @@ using Perspex.Controls.Primitives;
 using Perspex.Controls.Templates;
 using Perspex.Input;
 using Perspex.Interactivity;
+using Perspex.LogicalTree;
 using Perspex.Threading;
 
 namespace Perspex.Controls
@@ -92,6 +93,7 @@ namespace Perspex.Controls
         {
             SelectableMixin.Attach<MenuItem>(IsSelectedProperty);
             FocusableProperty.OverrideDefaultValue<MenuItem>(true);
+            IconProperty.Changed.AddClassHandler<MenuItem>(x => x.IconChanged);
             ItemsPanelProperty.OverrideDefaultValue<MenuItem>(DefaultPanel);
             ClickEvent.AddClassHandler<MenuItem>(x => x.OnClick);
             SubmenuOpenedEvent.AddClassHandler<MenuItem>(x => x.OnSubmenuOpened);
@@ -391,6 +393,26 @@ namespace Perspex.Controls
             }
         }
 
+        /// <summary>
+        /// Called when the <see cref="Icon"/> property changes.
+        /// </summary>
+        /// <param name="e">The property change event.</param>
+        private void IconChanged(PerspexPropertyChangedEventArgs e)
+        {
+            var oldValue = e.OldValue as ILogical;
+            var newValue = e.NewValue as ILogical;
+
+            if (oldValue != null)
+            {
+                LogicalChildren.Remove(oldValue);
+            }
+
+            if (newValue != null)
+            {
+                LogicalChildren.Add(newValue);
+            }
+        }
+
         /// <summary>
         /// Called when the <see cref="IsSubMenuOpen"/> property changes.
         /// </summary>

src/Perspex.Controls/Mixins/ContentControlMixin.cs

@@ -64,12 +64,19 @@ namespace Perspex.Controls.Mixins
 
                         var logicalChildren = logicalChildrenSelector(sender);
                         var subscription = presenter
-                            .GetObservable(ContentPresenter.ChildProperty)
+                            .GetObservableWithHistory(ContentPresenter.ChildProperty)
                             .Subscribe(child => UpdateLogicalChild(
                                 sender,
                                 logicalChildren, 
-                                logicalChildren.FirstOrDefault(), 
-                                child));
+                                child.Item1, 
+                                child.Item2));
+
+                        UpdateLogicalChild(
+                            sender,
+                            logicalChildren,
+                            null,
+                            presenter.GetValue(ContentPresenter.ChildProperty));
+
                         subscriptions.Value.Add(sender, subscription);
                     }
                 }