Improvements according to remarks and todo list

* introduced new setting $smwgFallbackSearchType holding the search engine to fall back to in case the search term is not a valid SMW query
* added a class alias SMWSearch for SMW\MediaWiki\Search\Search to simplify setting $wgSearchType in LocalSettings.php
* fixed a bug where SMWSearch::searchTitle() would fall back to searchText()
* added getter and setter for the DB connection used to find the standard fallback search engine
* improved some comments