Adds a search engine to allow semantic queries directly from MW's standard search

Activate it by setting in LocalSettings.php:
  $wgSearchType = 'SMWSearch';

This search engine will try to interpret the search term as a semantic query.

If succesful, the pages according to the query will be returned.
If not it falls back to the default search engine.

A custom fallback search engine different from the standard search engine can be set using $smwgFallbackSearchType. This is basically a replacement for $wgSearchType that is now used for SMWSearch