Move the implementation, of which most was in tl_cue.c, to demux_cue.c.
Currently, this is illogical, because tl_cue.c still accesses MPContext.
This is going to change, and then it will be better if everything is in
demux_cue.c. This is only a separate commit to distinguish code movement
and actual work; the next commit will do the actual work.