Draft: [#3052574] Facets AJAX

Combine all of the compatible contributions into a working branch to allow easy review and continued work on the issue.

Please "fix conflicts+rebase 2.0.x" instead of merging in 2.0.x to avoid breaking history to a point that automated rebase no longer works.

tests/src/FunctionalJavascript/AjaxBehaviorTest.php

@@ -101,11 +101,11 @@ class AjaxBehaviorTest extends JsBase {
     $assert_session->pageTextContains('Displaying 5 search results');
 
     // Check that the article_category option disappears when filtering on item.
-    $dropdown_entry = $this->xpath('//*[@id="block-duck-block"]/div/select/option[normalize-space(text())=:label]', [':label' => 'article_category']);
+    $dropdown_entry = $this->xpath('//*[@id="block-duck-block"]/div/div/select/option[normalize-space(text())=:label]', [':label' => 'article_category']);
     $this->assertNotEmpty($dropdown_entry);
     $block_owl->clickLink('item');
     $assert_session->assertWaitOnAjaxRequest();
-    $dropdown_entry = $this->xpath('//*[@id="block-duck-block"]/div/select/option[normalize-space(text())=:label]', [':label' => 'article_category']);
+    $dropdown_entry = $this->xpath('//*[@id="block-duck-block"]/div/div/select/option[normalize-space(text())=:label]', [':label' => 'article_category']);
     $this->assertEmpty($dropdown_entry);
 
     // Click the item facet again.
@@ -113,7 +113,7 @@ class AjaxBehaviorTest extends JsBase {
     $assert_session->assertWaitOnAjaxRequest();
 
     // Select the article_category in the dropdown.
-    $dropdown = $this->xpath('//*[@id="block-duck-block"]/div/select');
+    $dropdown = $this->xpath('//*[@id="block-duck-block"]/div/div/select');
     $dropdown[0]->selectOption('article_category');
     $assert_session->assertWaitOnAjaxRequest();