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.**
Because it only gets the pull from master that's why we allow to have it fail. It tests composer build on the lowest  denominator namely MW 1.19. If we would merged this with master it would complain as well.