Customizer Menus: custom metaboxes added by plugins are not displayed

[Chouby]

In a plugin, I use the function add_meta_box() in a function hooked to the action 'admin_init' to add a custom metabox in Appearance->menus. This metabox is not rendered when I want to add new menu items from the customizer.

[swissspidy]

We could add the customizer sections in the add_meta_box function, but that seems wrong to me. Perhaps plugins should need to do this separately, as meta boxes are very different to customizer sections/panels.

[celloexpressions]

At this point I think we've decided against using do_accordion_sections() here, since we have lazy-loading and other things going on here now. See #32708, and please test that patch/comment if you have additional needs that the proposed hooks there don't facilitate.