New function: Return true if bag contains a parameter-value-pair

* VuFindSearch/ParamBag.php (contains): New function. Return true if bag contains a parameter-value-pair.
Set offset and limit in parameter bag

* VuFindSearch/Backend/Solr/Backend.php (search): Set offset and limit
  in parameter bag.
* VuFindSearch/Backend/Solr/Connector.php (search): Change function signature,
  offset and limit contained in parameter bag.
Factor out spellcheck aggregation, use Connector::search()

* VuFindSearch/Backend/Solr/Backend.php (aggregateSpellcheck): New
  function. Factored out from search().
  (search): Factored out aggregated spellcheck, use local boolean var to
  indicate spellcheck.
Remove Connector::resubmit() et al

* VuFindSearch/Backend/Solr/Connector.php (resubmit, getLastQueryParameters,
  $lastQuery): Removed.
  (query): Remove usage of $lastQuery.
New function: Trigger error event

* VuFindSearch/Service.php (triggerError): New function. Trigger error
  event.
Catch BackendException, trigger error event & rethrow

* VuFindSearch/Service.php (search, retrieve,
  retrieveBatch) (similar): Catch BackendException, trigger error
  event & rethrow.
Add tagging support to backend exceptions

* VuFindSearch/Backend/Exception/BackendException.php ($tags): New
  property. Tags.
  (addTag, getTags, hasTag): New functions. Tagging support.