MKVToolNix 39.0.0 (32-bit)

What's new in this version:

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. Part of the implementation of 2486
- 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. Part of the fix of #2642
- MKVToolNix GUI: header editor: attachments with an empty name element will be shown as `<unnamed>` as originally intended. Part of the fix of #2642
- Linux AppImage: the AppImage will no longer change directories before running the desired executable allow the use of relative file names. Fixes #2632

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