BackendInterface.php: Accept ParamBag with backend parameters

* VuFindSearch/Backend/BackendInterface.php: Modify function
signature, accept ParamBag with backend parameters.
Modify according to BackendInterface

* VuFindSearch/Backend/Solr/Backend.php: Modify function signature
according to BackendInterface.
(search): Adjust check of spellcheck enabled/disabled.
Accept backend parameters in Solr/Connector search operations

* VuFindSearch/Backend/Solr/Connector.php: Accept backend parameters
in search operations.
Only use query to build query in Solr/QueryBuilder

* VuFindSearch/Backend/Solr/QueryBuilder.php (build): Only use query
to build query.
Modify functions and function signatures to fit new BackendInterface

* VuFindSearch/Service.php: Modify functions and function signatures
to fit new BackendInterface.
Add temporary getter for raw SOLR response

* VuFindSearch/Backend/Solr/Response/Json/RecordCollection.php
(getRawResponse): Add temporary getter for raw SOLR response.