core: runtime Matroska edition switching

Add a new slave property which switches the current Matroska edition.
Since each edition can define an entirely new timeline, switching the
edition will simply restart playback at the beginning of the file with
the new edition selected.

Add 'E' as new keybinding to step the edition property.

DVD titles are still separate. Apparently they work similarly, but I
don't have any multi-title DVDs for testing. Also, cdda (for audio CDs)
uses the same mechanism as DVDs to report a number of titles, so there
seems to be confusion what exactly this mechanism is supposed to do.
That's why the edition code is completely separate for now.

Remove demuxer.num_titles. It was just a rather useless cache for the
return value of the DVD titles related STREAM_CTRL.

One rather obscure corner case isn't taken care of: if the ordered
chapters file has file local options set, they are reset on playback
restart. This is unexpected, because edition switching is meant to
behave like seeking back to the beginning of the file.

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