fix broken relative mode in ASSLineContents:callback()/:modTags()/getTags()/removeTags() and ASSLineTagSection:callback(); fix missing parameter in ASSLineTagSection:modTags()
support negative start and end ranges in ASSLineContents:callback() and ASSLineTagSection:callback(); remove confusing "relative" parameter from ASSLineContents:insertTags() and introduce direct mode in its stead