Added support for Kobo's "series" organization feature as well. Apparently requires some janky 2-stage process because it's not done with the book's built-in metadata; you have to sync the books to the Kobo, then sync again for it to poke the special series sauce after the files are on the device.

Seems like a needlessly complicated way to do it when the books have series metadata, so props to Calibre for the extra work to support that!