Log Message:
mkvtoolnix: updated to 31.0.0

Version 31.0.0 "Dolores In A Shoestand":

New features and enhancements
* all programs: added a new option --abort-on-warnings that will cause the
  program to abort after it has emitted the first warning, similar to how it
  aborts after the first error.
* mkvmerge, mkvextract: when closing files that were opened for writing,
  cached data will not be flushed to storage automatically anymore. This
  reverts the workaround implemented for 2469. A new option was added to both
  programs (--flush-on-close) that re-enables flushing for people who are
  affected by data loss such as described in 2469.

  The reason is that automatic flushing causes long delays in processing
  queues when the output by mkvmerge/mkvextract isn't the final product but
  just an intermediate result to be processed further.
* MKVToolNix GUI: multiplexer: the dialog previewing different character sets
  for text subtitles will now keep the position of the displayed text when
  switching between character sets.

Bug fixes
* mkvmerge: AVI reader: using DV type 1 AVIs will now result in an unsupported
  file type being reported (as the underlying AVI library doesn't support
  them) instead of crashing mkvmerge.
* mkvmerge: HEVC: the height of interlaced streams will now be set correctly
  to the height of the full frame instead of the height of a single interlaced
  field.
* mkvmerge: MP4 reader: edit lists consisting solely of elements that mkvmerge
  doesn't support (such as dwells) are simply ignored. Before no data was read
  for such tracks at all.
* mkvmerge: text subtitles: entries with an explicit duration of 0ms will now
  be handled correctly: the 0ms duration will be stored in Matroska instead of
  the difference between the current and the following entry.
* MKVToolNix GUI: multiplexer, chapter editor: fixed drag & drop handling with
  Qt 5.12.0 and newer.
* MKVToolNix GUI: multiplexer: the GUI did not clean up temporary files
  created when running mkvmerge.

Build system changes
* Qt 5.4.0 or newer has required (up from 5.3.0) since version 30.0.0; I just
  forgot to include this entry.