Fixed: a disabled CPMenuItem could still have a selected background

Previously it was possible to have a highlighted CPMenuItem for a disabled CPMenuItem. (To get that, select an item, close the menu, disabled the item manually and then open the menu again).
Now it is not possible to get this behavior.

AppKit/CPMenuItem/CPMenuItem.j

@@ -166,6 +166,9 @@ var CPMenuItemStringRepresentationDictionary = @{
     if (_isEnabled === isEnabled)
         return;
 
+    if (!isEnabled && [self isHighlighted])
+        [_menu _highlightItemAtIndex:CPNotFound];
+
     _isEnabled = !!isEnabled;
 
     [_menuItemView setDirty];