Log Message:
mkvtoolnix: updated to 39.0.0

Version 39.0.0 "In The Waiting Line":

New features and enhancements

* mkvmerge: Blu-ray: when reading an MPLS playlist mkvmerge will look up and
  use chapter names from the Blu-ray's "track/chapter names" meta data \ 
if it
  exists.
* mkvmerge: MP4 reader: if present, cover art images (the
  `meta.udta.ilist.covr` atom) will be converted into attachments.
* MKVToolNix GUI: multiplexer: when adding a playlist from a Blu-ray disc, the
  disc library meta data will be parsed, and the biggest thumbnail, if
  present, will be added as a new attachment with name `cover.jpg` (extension
  depends on thumbnail's extension).
* MKVToolNix GUI: multiplexer: when adding a playlist from a Blu-ray disc, the
  title from the disc library meta data will be set as the new file title if
  the disc library meta data contains one & no title has been set yet.
* MKVToolNix GUI: multiplexer: the automatically generated destination file
  name will now be based on the file title if one is set at that point. This
  works in conjunction with the title being said from the Blu-ray disc library
  meta data.
* MKVToolNix GUI: chapter editor: when reading chapters from an MPLS playlist
  the GUI will look up and use chapter names from the Blu-ray's "track/chapter
  names" meta data if it exists. Part of the implementation of 2486.
* MKVToolNix GUI: Windows: added a dark mode that's enabled when Windows 10's
  dark mode is turned on.
* translations: added a Bulgarian translation of the programs & the man pages
  by Симеон Цветков (see `AUTHORS`).

Bug fixes

* mkvmerge: attachments without a file name won't be ignored anymore.
* MKVToolNix GUI: header editor: attachments with an empty name element will
  be shown as `<unnamed>` as originally intended.
* Linux AppImage: the AppImage will no longer change directories before
  running the desired executable allow the use of relative file names.

Build system changes

* MKVToolNix now requires a C++ compiler that supports the following features
  of the C++17 standard: "`[[maybe_unused]]` attribute", "nested \ 
namespace
  definition", "structured bindings". For the GNU Compiler \ 
Collection (gcc)
  this means v7 or newer; for clang it means v4 or newer.
* Boost 1.60.0 or newer is now required.