Widget Customizer: Show all widgets with message indicating which aren't on the page

[JoshuaWold]

This ticket follows up on the discussion started by @shireling on #39087.

The problem: The customizer only displays the widgets for the page you're on. For consistency and usability, we'd like to have all widgets appear in the customizer.

Solution: Show all widgets in the customizer with a message indicating which aren't showing on the page. See attachment for suggest solution.

[westonruter]

What if the notification there is also accompanied by (or part of) and accordion bar that could show/hide the widget areas below? By default it could be expanded, but collapsing it could set a user preference to remember the expanded state.

[westonruter]

An accordion to show/hide the inactive sidebars would actually have some parity with what is proposed in #27404 for adding inactive widgets to the available widgets panel.

[zoonini]

This is also being discussed (in a slightly broader context) in #40432

[westonruter]

@zoonini ah, right! Do you think this should be marked as a duplicate and the scope of the other ticket focus on widgets?

[JoshuaWold]

Duplicate of #40432.

Good catch. Marking this as duplicate. Thanks!