Marked todo related to child priority sorting.

Source/Urho3D/UI/UIElement.cpp

@@ -1626,6 +1626,7 @@ void UIElement::SortChildren()
     if (sortChildren_ && sortOrderDirty_)
     {
         // Only sort when there is no layout
+        /// \todo Order is not stable when children have same priorities
         if (layoutMode_ == LM_FREE)
             Sort(children_.Begin(), children_.End(), CompareUIElements);
         sortOrderDirty_ = false;