ExtraLinks should use route title, instead of hardcoded title

Problem/Motivation

I've customized route titles, but Admin Toolbar does not show them.

Steps to reproduce

Implement a route subscriber to customize a route provided by \Drupal\admin_toolbar_tools\Plugin\Derivative\ExtraLinks::getDerivativeDefinitions (such as a Field UI route)

Proposed resolution

If we're calling \Drupal\admin_toolbar_tools\Plugin\Derivative\ExtraLinks::routeExists, actually return the route and use its title.

This might be difficult if it uses a title callback, though.

Remaining tasks

User interface changes

API changes

Data model changes