Support for menu attributes

As I understand the settings we are talking about are NOT stored in the menu item itself? There are some hooks that allow you to inject any information you want into menu item JSON while exporting and then parse it back during the import and take relevant actions. I don't plan to add any special support for other contribs right now.

I checked the way menu_attributes work - it stores its data in variables. That means you can use features to export menus and their attributes. Also menu_import is not supposed to care about menu settings, but rather about menu items transfer from one instance to another.