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.**
> not work with the Virtuoso store? Or is the Virtuoso store just buggy?

Not really, the SPARQL will look like:
```
wiki:Some_string_that_can_be_interpreted_as_a_semantic_query swivt:wikiPageSortKey ?resultsk .
{ wiki:Some_string_that_can_be_interpreted_as_a_semantic_query swivt:wikiNamespace "0"^^xsd:integer . }
```

Since both external stores are empty, Fuseki returns an empty xml which creates an empty `QueryResult` object. In case of Virtuoso, a `false` is returned instead of an xml which of course is invalid for the `RawResultParser`.

I will probably change the `RawResultParser` to allow `false` to be a valid result as well.