MKVToolNix 55.0.0 (64-bit)

What's new in this version:

Fixed:
- All programs, only on Windows: fixed checking existence of & creating directories with UNC paths such as `\serversharefolderfile.mkv`
- All programs, only on Windows: implemented several workarounds for bugs in `std::filesystem` wrt. UNC paths. The effect of those bugs was that Blu-rays and DVDs couldn't be added from UNC paths.
- Mkvmerge, only on Windows: fixed non-ASCII characters getting mangled in the destination file name when splitting is active
- MKVToolNix GUI, only on Windows: fixed the GUI hanging when trying to browse for files on drives that don't actually exist
- MKVToolNix GUI, only on Windows: the GUI did not remember UNC paths as the "most recently used source directory" properly
- MKVToolNix GUI: only on Windows: when dragging & dropping files from other applications to MKVToolNix GUI the GUI will force the drop action to be a copy action, no matter which keys were pressed by the users. This prevents Windows Explorer from deleting the dragged file when the user presses shift while dropping the file onto MKVToolNix GUI.
- MKVToolNix GUI, only on Windows: when using the file dialog for selecting a file name to save to (e.g. when browsing for the destination file name in the multiplexer or saving an attachment in the header editor) non-ASCII characters in the default file name were mangled.
- MKVToolNix GUI: multiplexer: the user can now configure what to do when using the "Add source files" button independently of the setting for what happens when files are dragged & dropped onto the GUI. The same options are available, but they're distinct settings, allowing for different use cases.
- MKVToolNix GUI: multiplexer: deriving the track language from file names doesn't use one big regular expression of all enabled languages anymore. Instead, the file name is split into parts on a list of character (such as `.` or `(` and `)`; configurable in the preferences). This prevents the regular expression becoming too big for the regular expression library to handle when many languages are enabled.
- MKVToolNix GUI: preferences: the language lists for "deriving track languages" and "enabling items by language" always included all the ISO 639-3 languages, even if those weren't enabled in the "often-used languages" pane.

Changes:
- Mkvmerge, MKVToolNix GUI multiplexer: the functionality for changing the length of the NALU "size" field for AVC/H.264 and HEVC/H.265 tracks has been removed. The corresponding command-line option will continue to be recognized by mkvmerge as not to break existing third-party applications, but it won't actually do anything.