Finish PhocInput gobjectification

In !298 (merged) i thought I'd be done with cleanups but that one glared at me at couch time.

Using a GSList doesn't add much code and since we don't add many members the fact that this needs an allocation shouldn't be that bad. I kept a list rather than using a hash keyed by wlr_seat (which would help phoc_input_seat_from_wlr_seat since the list is usually really one item.