Solrutils: new query spelling corrector

 * Corrects Solr queries
WebSearch: new query term corrector

 * Calculates nearest terms more accurately (addresses #1176)
WebSearch: refactored query corrector

 * Splits source code in multiple classes

 * Allows to reuse caches for phrase correction
WebSearch: new nearest terms output box

 * Integrates native and Solr query spelling correctors