Port Views integration

Wow, our first trait, great!
Also, great work otherwise, looks very good! And since I already saw it work, even better!
Just a few remarks:

1. +++ b/search_api.api.php
   @@ -319,5 +319,34 @@ function hook_search_api_index_reindex(\Drupal\search_api\Index\IndexInterface $
     */
   +
   diff --git a/search_api.views.inc b/search_api.views.inc
   

   Superfluous newline.

2. +++ b/src/Entity/Index.php
   @@ -1084,6 +1084,16 @@ class Index extends ConfigEntityBase implements IndexInterface {
   +          $this->invalidateViewsCache();
   +        }
   +        // Check whether the indexed fields changed.
   +        if ($this->original->getFields() != $this->getFields()) {
   +          views_invalidate_cache();
   

   Is there a reason why you use the function directly, instead of the method, in the latter case?

3. +++ b/src/Entity/Index.php
   @@ -1172,10 +1182,32 @@ class Index extends ConfigEntityBase implements IndexInterface {
   +   * Function for reacting to a disabled or deleted search index.
   +   */
   +  protected function invalidateViewsCache() {
   

   Is this correct? Wouldn't it allow the user to create a new view for a deleted index after an index has been deleted (or with fields not available anymore)? Or is the Views cache automatically cleared when creating a new view (or enabling a disabled one)?

4. +++ b/src/Plugin/views/argument/SearchApiArgument.php
   @@ -0,0 +1,154 @@
   + * @file
   + * Contains SearchApiViewsHandlerArgument.
   

   Needs to be namespaced, and name is not up to date.
   Also, why does this need a module prefix? Is there some special rule, or only because "Argument" alone would sound stupid? Maybe "SearchArgument" then?

5. +++ b/src/Query/QueryInterface.php
   @@ -165,7 +165,9 @@ interface QueryInterface {
   -   *     the items' IDs, values are arrays containing the following keys:
   +   *     the concatenated datasource and the ID, the delimiter is
   +   *     IndexInterface::DATASOURCE_ID_SEPARATOR, values are arrays containing
   +   *     the following keys:
   

   This shouldn't be documented in every single instance where IDs are used. We should probably give some thought to where to best explain this, but for now just use "internal item IDs", or an additional "(with datasource ID)" if you want to clarify.
   But in general, we use these combined IDs nearly everywhere now, except in the datasource itself (and some index code dealing with datasources, of course), so even that clarification shouldn't be necessary.

6. +++ b/src/Tests/ExampleContentTrait.php
   @@ -0,0 +1,223 @@
   +/**
   + * Contains \Drupal\search_api\Tests\TestDataTrait.
   + */
   

   Missing "@file" and correct name.

7. +++ b/src/Tests/ExampleContentTrait.php
   @@ -0,0 +1,223 @@
   +
   +} ¶
   diff --git a/src/Tests/SearchApiViewsTest.php b/src/Tests/SearchApiViewsTest.php
   

   Trailing space.

8. +++ b/tests/modules/search_api_test_backend/search_api_test_backend.info.yml
   @@ -2,8 +2,7 @@ name: 'Search API Test Backend'
   -version: VERSION
   

   This is also used in libraries.yml, and I already wondered. Is it correct there?