Fujifilm X-Series updates

- handle 14bpp RAFs (currently produced by X100S, X-E2, and X-T1 cameras)
- add/update X-Series camera definitions
- pedantically alter cameras.xml DTD (apologies if this is wrong/misconstrued!)

This pull request is on the suggestion pmjdebruijn. Changes have been tested against RAFs from all X-Series cameras via https://github.com/dtorop/darktable/tree/xtrans2 which is under review as https://github.com/darktable-org/darktable/pull/588.