Hide default local tasks is broken (was: Test fail)

There was 1 failure:
1) Drupal\Tests\sticky_local_tasks\Functional\StickyLocalTasksHideDefaultTest::testDefaultLocalTasks
Behat\Mink\Exception\ElementNotFoundException: Element matching xpath "//link[contains(@href, "/sticky_local_tasks/assets/css/hide_default_local_tasks.css")]" not found.
/builds/project/sticky_local_tasks/vendor/behat/mink/src/WebAssert.php:465
/builds/project/sticky_local_tasks/tests/src/Functional/StickyLocalTasksHideDefaultTest.php:45

It looks like this might've been introduced in 38557cb8689c0ac8a48711565f5224939e398cca. That removed the hide-default-local-tasks library (although it still gets conditionally attached in \Drupal\sticky_local_tasks\StickyLocalTasksBuilder::build()).

Also the config setting was removed in ec7ba618eabf0e2b97e03cec1f3f5af04fd99fbc so the initial part of the test would fail because it expects the hidden setting to be enabled by default.

As an aside, I was wondering if it might make sense to use a javascript test and check the actual visibility of the links rather than the presence of a CSS file?

Thanks @jan kellermann. I've done a code review but haven't tested it yet. Because we released 2.0.2 without the config setting usage_options.hide_default_local_tasks set to true, it's possible some users have installed that version and haven't got the config correctly set. I think we should also add a hook_update_N() function that checks if the key exists, and if not adds it and sets it to true. I'm happy to have a go at this, but won't be able to take a look until tomorrow at the earliest.

Looks great, thanks! Reviewed and tested, the hook update works, toggling the config option actually affects the display of the tasks, and the config validates. Just one super minor nitpick over a comment...