MKVToolNix v75.0.0 “Goliath”

# Version 75.0.0 "Goliath" 2023-03-26

## New features and enhancements

* mkvmerge: MP4 reader: mkvmerge will now read Timed Text (FourCC `tx3g`)
  subtitle tracks & convert them on the fly to Matroska's simple text subtitle
  format (`S_TEXT/UTF8`; text only, no styles). Implements #2208, #2242,
  #2613, #3000, #3243, #3418 and possibly others.

## Bug fixes

* all: switched back to using `boost::filesystem` functions for creating
  directories instead of the ones introduced to work around bugs in
  `std::filesystem`. The latter didn't work correctly with UNC paths after the
  switch to `boost::filesystem::path` in v74. Fixes #3483.
* mkvmerge: VobSub reader: mkvmerge will now probe the `.idx` file during
  VobSub identification even if the `.sub` file is passed as the
  source. Avoids mis-detection of the `.sub` as MPEG program streams. Fixes
  #3489.
* MKVToolNix GUI: the GUI could abort with an exception on startup while
  looking for the `mkvmerge` or `mediainfo` executables due to inaccessible
  folders. Fixes #3481.
* MKVToolNix GUI: preferences: when opening the preferences the first time the
  UI might pre-select the first entry in the list of interface languages if
  the operating system's language is not available for MKVToolNix. This might
  also happen on Linux if e.g. `en_GB` is set, even though `en_US` is
  available. Now English (`en_US`) will be selected instead. Fixes #3486.
* MKVToolNix GUI: multiplexer: when adding files the GUI has special handling
  for chapter/tags/segment info files. This is done by comparing their content
  to certain patterns. This recognition could wrongfully be triggered if any
  such file was embedded in another file verbatim, e.g. with a chapter XML
  file attachment in a Matroska file. When trying to add that Matroska file,
  the GUI would treat it as a chapter file instead of a regular one. This
  content-based detection was fixed. Fixes #3487.

## Other changes

* mkvpropedit, GUI's header editor: removed support for the deprecated
  "minimum cache" & "maximum cache" track header elements.