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

**(DO NOT MERGE YET. FOR DISCUSSION ONLY.)**

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

**Todo:**
* Tests
* RELEASE-NOTES (once SMW 2.0 is out)

**Any comments (in particular regarding architecture) welcome.**
DB=mysql; MW=1.23.0; VIRTUOSO=6.1 fails with:

```
1) SMW\Tests\MediaWiki\Search\SearchTest::testSearchTitle_withSemanticQuery
RuntimeException: Parser error: Not well-formed (invalid token)
/home/travis/build/SemanticMediaWiki/mw/extensions/SemanticMediaWiki/includes/src/SPARQLStore/QueryEngine/RawResultParser.php:106
/home/travis/build/SemanticMediaWiki/mw/extensions/SemanticMediaWiki/includes/src/SPARQLStore/GenericHttpDatabaseConnector.php:440
/home/travis/build/SemanticMediaWiki/mw/extensions/SemanticMediaWiki/includes/src/SPARQLStore/GenericHttpDatabaseConnector.php:234
/home/travis/build/SemanticMediaWiki/mw/extensions/SemanticMediaWiki/includes/src/SPARQLStore/QueryEngine/QueryEngine.php:217
/home/travis/build/SemanticMediaWiki/mw/extensions/SemanticMediaWiki/includes/src/SPARQLStore/QueryEngine/QueryEngine.php:141
/home/travis/build/SemanticMediaWiki/mw/extensions/SemanticMediaWiki/includes/src/SPARQLStore/SPARQLStore.php:264
/home/travis/build/SemanticMediaWiki/mw/extensions/SemanticMediaWiki/includes/src/MediaWiki/Search/Search.php:134
/home/travis/build/SemanticMediaWiki/mw/extensions/SemanticMediaWiki/tests/phpunit/includes/MediaWiki/Search/SearchTest.php:149
```