\SMW\ParserOutputGenerator

The $this->wikiPage->getParserOutput() approach introduced with I3b41d4e506a
did not yield sufficient results and furthermore for MW 1.19 we need to
reparse the text using $wgParser->parse() while in MW 1.21
$revision->getContent( Revision::RAW ) can be used.

Instead of introducing $wgParser again to the UpdateJob we use a
dedicated class that handles the generation of ParserOutput object.

Change-Id: Id8c5e83ed992afd8984feaa153e2bd46cfa06484