Doxygen documentation update:
* Changed alls @addtogroup to @ingroup. @addtogroup adds the comment to the group description, but doesn't add the file, class, function, ... to the group like @ingroup does. See for example http://svn.wikimedia.org/doc/group__SpecialPage.html where it's impossible to see related files, classes, ... that should belong to that group.
* Added @file to file description, it seems that it should be explicitely decalred for file descriptions, otherwise doxygen will think that the comment document the first class, variabled, function, ... that is in that file.
* Removed some empty comments
* Removed some ?>
Added following groups:
* ExternalStorage
* JobQueue
* MaintenanceLanguage
One more thing: there are still a lot of warnings when generating the doc.
Patch by MinuteElectron - https://bugzilla.wikimedia.org/attachment.cgi?id=4772 with fixes
Replaces an instance variable with a constant, since it doesn't change, which is safe to access when the function is called statically from an outside class, as some extensions do.
* Verified Preprocessor_Hash against Preprocessor_DOM with a differential fuzz test.
* Improved the utility of Parser_DiffTest by synchronising strip markers and allowing for loose object comparison.
* Abstracted PPDStack to make it usable in Preprocessor_Hash.
