Add WordPress widget name as the resource

[morrisonlevi]

Description

Reworks the widgets section of the WordPress integration:

• Drops WP_Widget::display_callback and instead trace overrides of the WP_Widget::widget method, and use the WP_Widget::$name as the resource.
• Drops WP_Widget_Factory::_register_widgets for its parent call wp_widgets_init. This is a minor change.

Here's what it looked like before:

And after:

It's much easier to see the widget responsible for the queries now.

Readiness checklist

• [ ] Changelog has been added to the release document.
• [ ] Tests added for this feature/bug.

Reviewer checklist

• [x] Appropriate labels assigned.
• [x] Milestone is set.