[UX] Reduce the size of the potentially unwieldy parent link selector on content type and content edit pages

This is a follow-on to https://github.com/backdrop/backdrop-issues/issues/6390. In that issue, on the menu add/edit link page, the "Parent link" selector contains almost all possible menu items, and so is large and unwieldy. That issue solved the problem by adding a separate selector to choose the menu, and then the 'Parent link" selector only contains choices from that menu.

It was requested there by @klonos:

...also, can we please extend the scope of this to also add the same behavior in the "Menu settings" vertical tab of content types as well as the content edit form? (when more than one menu has been configured to be available for a content type)

@bugfolder replied:

It looks like the vertical tab of content types already has this behavior (or perhaps I'm misunderstanding what you're talking about). Here's the content type edit page with two menus selected, and only those two menus' items are being shown in the default parent link:

And here's the content edit form, which shows the same thing:

@klonos replied:

...perhaps I didn't provide enough information to make it clear. If you enable all checkboxes for all available menus, including the admin menu, then the parent menu selection becomes unwieldy again.

So OK, allowing content links to be added to the admin bar is an edge case, however I used the admin menu as an out-of-the-box example which is easy to test. You could generate a fresh, rather huge menu with devel (or a few smaller menus and enable all of them). Then the problem we are trying to solve here is present in the content type configuration form, as well as the content edit form.

We'll continue the discussion from here.