Customizer Menus: add a way to extend available menu items

[designsimply]

Moving this over from ​https://github.com/voldemortensen/menu-customizer/issues/103

From @sevenspark:

Can we add a way to extend available menu items?

In the Appearance > Menus screen, we can use add_meta_box to insert custom menu item types.

However, in available_items_template(), there isn't the ability to add any additional types of menu items (though I see there is an @todo note regarding add_meta_box)

Right now, the Menu Customizer doesn't interfere with any custom items added in Appearance > Menus as far as I can tell, so that's good. But it'd be nice to be able to add them via the Customizer interface as well.

I think just adding an action hook would probably be all we'd need.

Thanks! :)

[designsimply]

Also copying in @westonruter's previous comment from ​https://github.com/voldemortensen/menu-customizer/issues/103#issuecomment-111257349 because it's worth noting:

Yes, this is something that needs to be added at some point. I have an initial TODO for where we can pick up additional controls added: ​https://github.com/WordPress/WordPress/blob/f5fa907f77ea0b3e1035de96c6de6c1be37b4688/wp-admin/js/customize-nav-menus.js#L1061

What comes to mind to implement this is:

1. Ability to add new controls to the template.
2. Connect the new controls to the setting, with custom validation logic.
3. Add support for filtering the postmeta for a given nav_menu_item so that any custom fields will get previewed as expected.
4. Update the WP_Customize_Nav_Menu_Item_Setting to store any non-core attributes in postmeta, and draw from postmeta when obtaining the setting value.

The first two can be done in JS now, although there isn't a nice API for doing it.

[ocean90]

Duplicate of #32708.