Added QueryInterface::getOriginalQuery()

Up to now, there was no (simple) way to re-execute an already executed search query. The results are statically cached, so calling execute() again will just return the same result set as before, without any query.

This shortcoming in the API was now amended by the introduction of the \Drupal\search_api\Query\QueryInterface::getOriginalQuery() method, which can be used to retrieve a copy of the unexecuted query. The copy is made before preExecute() runs, so the preprocessing can also be repeated (which might be necessary for some changes to the query to have the desired effect).