Customize: Nav menu item controls should be inactive when items are excluded from preview

[westonruter]

Plugins can exclude nav menu items from displaying in nav menus in a plugin like Nav Menu Roles. This is similar to how widgets can be conditionally excluded from sidebars via a plugin like Jetpack's Widget Visibility module. This is currently implemented for widgets, where a widget that is not shown in the preview will have its control's active state set to false (causing it to become partially-opaque rather than fully hidden). Something similar should be done for nav menu item controls that are excluded from the preview.

[westonruter]

39091.wip.diff​ almost gets us there, except it is then not possible to expand the nav menu item when it is inactive.

[celloexpressions]

This would be a good usability improvement.