Allow custom post type menu registrations to be submenus instead of top-level if desired

[jane]

Right now, registering a custom post type creates a top-level admin menu for it. However, in some cases it may make more sense for the custom post type to be a submenu item. For example, if someone creates a custom post type for special video content, podcasts, etc, it might make more sense for it to live under Media, to keep the left menu tidy (and less of a problem when menus are expanded on shorter screens).

[johnbillion]

#14145