Encode File/Info/Subtitle in the titleStmt

Note: not encoding it in work description just now, but can add it, if you prefer duplicating the info. Due to the abstract nature of workDesc, I think it is arguable whether sibmei should create this element at all. Does the information in the File/Info fields have to do with the work as an abstract entity, or with the specific Sibelius file by which it is represented?