Move addTrackingCategory from Parser to ParserOutput

addTrackingCategory is more in line with ParserOutput's functionality
(addLink, addCategory etc), and tracking categories are useful even for
content types which do not use the parser at all. There is no reason to
require the caller to obtain a Parser object just to be able to add
tracking categories.

Change-Id: I89d9ea1db3a4e6486e77eee940bd438f7753b776
Merge "Move addTrackingCategory from Parser to ParserOutput"