MKVToolNix 91.0 “Signs”

# Version 91.0 "Signs" 2025-03-16

## Important notes

* The MKVToolNix project's source code repository, bug tracker and wiki have
  been moved to [Codeberg](https://codeberg.org/mbunkus/mkvtoolnix).
* The customized MXE repository used for building the Windows variants has
  been moved from Gitlab to [Codeberg](https://codeberg.org/mbunkus/mxe).

## New features and enhancements

* MKVToolNix GUI: multiplexer: added actions in the "Multiplexer" menu for
  saving all currently open multiplexer tabs to a single `.mtxcfg`
  file. Loading this file later will restore all tabs. Implements #3797.
* MKVToolNix GUI: multiplexer: when adding files the GUI can automatically
  enable the "commentary" flag for audio and subtitle tracks if the file name
  contains the word "comments" or "commentary" delimited by certain characters
  (configurable). This feature is turned on by default. Part of the
  implementation of #3798.
* MKVToolNix GUI: multiplexer: when adding files the GUI can automatically
  enable the "original language" flag for audio and subtitle tracks if the
  track's language matches one configured by the user (configurable). This
  feature is turned off by default. Part of the implementation of #3798.
* MKVToolNix GUI: multiplexer: when adding files the GUI can automatically set
  several track flags based on the file name. It can now look for the same
  patters in the track names, too (configurable). This feature is turned on by
  default. Part of the implementation of #3798.
* MKVToolNix GUI: multiplexer: added special drag & drop zones where users can
  drop files to circumvent the preferences wrt. what to do with multiple
  dropped files. Each drop zone corresponds to one of the three possible
  choices (add all to current settings; create one new settings & add all
  there; add new settings for each file & add one file to each). This feature
  is turned off by default. Implements #3796.
* MKVToolNix GUI: multiplexer: added additional special drag & drop zones
  where users can drop files to add them as attachments. One of the zones will
  automatically set up the dropped file to function as a cover
  image. Implements #3794.
* MKVToolNix GUI: multiplexer: when making destination file names unique by
  suffixing them with a running number the GUI will now also consider the
  destination file names of multiplex jobs currently in the queue that are
  currently running or still waiting for execution. Implements #3801.

## Bug fixes

* mkvmerge: timestamps format v3: fixed the parser; it was completely broken
  for the lines containing the duration & an optional number of frames per
  second. Fixes #2285.
* mkvmerge: DTS parser: if the X96 extension is present in a DTS-HD High
  Resolution stream, the sampling frequency will be set to 96kHz. This was
  broken when reading DTS from elementary streams & from Matroska files. Fixes
  #3288.
* mkvpropedit, MKVToolNix GUI's chapter & header editors: fixed a bug leading
  to a segmentation fault when trying to replace an element at the end of the
  file that's followed by only void elements (at least one). Fixes #3802.

## Build system changes

* The bundled `nlohmann-json` library was updated to v3.11.3.