Allow auto-adding entire sub-menu to menu item

[joelworsham]

When adding a menu item, ask if the user wants to add the entire submenu as well. Here is the current workflow that is confusing:

1. Activate Client Dash and customize/save a menu.
2. Activate a plugin that adds menu items, let's say WooCommerce.
3. Go back to customizations and add "WooCommerce" menu item back in.
4. "WooCommerce" sub-menu is entirely empty and you must add each sub-menu item by hand

I'd like to allow removal of step 4 by auto-adding submenu items.