Customize: newly-added pages should show that they were added to the menu

[celloexpressions]

When adding new pages in menus, the pages should show as "checked" or added within the available nav menu items panel.

[celloexpressions]

Currently, the new items are added to the menu and the top of the available items list, but not shown as checked.

[celloexpressions]

We need to add the item-added class the the menu item handle in the newly-added item's template in the available nav menu items panel. Should be a one-line addition to customize-nav-menus.js.

[ryankienstra]

Working On A Patch

Hi @celloexpressions,
If it's alright, I'm working on this now. I'll have a patch ready soon.

[celloexpressions]

Awesome, thanks @ryankienstra!

[ryankienstra]

Request For Review

Hi @celloexpressions,
Could you please review the attached patch, or the identical ​GitHub pull request?

This passes the ​available-menu-item template a value of true for ​is_new_item. The template will then output the class item-added, as you suggested.

Background:
Adding a new page to a menu in the Customizer uses the template ​available-menu-item. Existing pages also use this template, so this can't simply output the class for all of them.

[ryankienstra]

An Approach With Only JavaScript

Hi @celloexpressions,
Thanks for ​reviewing this PR. Is this patch what you had in mind in your ​comment?

[celloexpressions]

Thanks @ryankienstra, alt-38499.diff​ looks ready for commit. It's a bit simpler with that approach I think.

[westonruter]

In 39002:

Customize: Mark newly-added page/post stubs as added (with checkmark) when they are inserted into the list of available items.

Props ryankienstra, celloexpressions.
See #34923.
Fixes #38499.