Use GET request for dropdown widget

To-do: ensure that there is only one active item. Configurable placeholder text?

Thanks for that patch Stijn!

We already have a processor that ensures only 1 item can be active, we should probably make sure that processor is enabled by default for this widget. I attached a patch that removes a useless unit test and updates a failing one. The integration test'll still fail.

I think the easiest way forward here is implementing #2611198: Give widgets the ability to require settings and it's inverse.

This makes the integration test pass, we could commit this, but I wouldn't feel very confident right now, without any test coverage. So I'd love to get #2719065: Javascript test for widget in first, so we can test the JS that makes this into a dropdown.

So, let's keep this patch as-is for now, and we'll come back here soon.

I broke the js, I think. But I don't know why.

Reverted #11.

Ran eslint over the new js file.

Eslint + phpcs fixes, changed 1 doc comment in js. RTBC'd the patch because I think it looks great. We can add test coverage next in #2719065: Javascript test for widget

Nits.

1. +++ b/facets.libraries.yml
   @@ -28,3 +28,13 @@ drupal.facets.checkbox-widget:
   +    - core/jquery.once
   \ No newline at end of file
   

   Needs a newline at the end of file.

2. +++ b/tests/src/Unit/Plugin/widget/DropdownWidgetTest.php
   @@ -0,0 +1,111 @@
   +  private function buildLinkAssertion($text, $count = 0, $active = FALSE, $show_numbers = TRUE) {
   

   Why is this private function? Probably protected function?

Committed, see you in #2719065: Javascript test for widget to add test coverage.

Oh, sorry about #22, didn't see that before I committed, I've fixed those in the attached patch, you agree with those changes?

Committed that followup patch.