Ordered chapter code tries opening files to find those matching the
SegmentUID values specified in the timeline. Previously this scan did
a full initialization of the Matroska demuxer for each file, then
checked whether the UID value in the demuxer was a match. Make the
scan code instead provide a list of searched-for UIDs to the demuxer
open code, and make that do a comparison against the list as soon as
it sees the UID in the file, aborting if there is no match.
Also fix units used in "Merging timeline part" verbose message.