This moves the implementation of ParserOutput::addTrackingCategory()
to the TrackingCategories class as a non-static method. This makes
invocation from ParserOutput awkward, but when invoking as
Parser::addTrackingCategory() all the necessary services are
available. As a result, we've also soft-deprecated
ParserOutput::addTrackingCategory(); new users should use the
TrackingCategories::addTrackingCategory() method, or else
Parser::addTrackingCategory() if the parser object is available.
The Parser class is already kind of bloated as it is (alas), but there
aren't too many callsites which invoke
ParserOutput::addTrackingCategory() and don't have the corresponding
Parser object handy; see:
https://codesearch.wmcloud.org/search/?q=%5BOo%5Dutput%28%5C%28%5C%29%29%3F-%3EaddTrackingCategory%5C%28&i=nope&files=&excludeFiles=&repos=
Change-Id: I697ce188a912e445a6a748121575548e79aabac6
