player: move timeline scanning (ordered chapters etc.) to a thread

Do timeline building (scanning & opening reference files for ordered
chapters, and more) in a thread. As a result, this process can actually
be stopped without having to kill the player.

This is pretty simple: just reuse the demuxer opening thread. We have
to give up on the idea that open_demux_reentrant() is reusable, though.

(Althoughthe timeline readers still need some fixes before they react to
the quit request.)