MKVToolNix v78.0 “Running”

# Version 78.0 "Running" 2023-07-02

## New features and enhancements

* MKVToolNix GUI: header editor: for files that only contain legacy track
  language elements but no IETF language elements the header editor will now
  derive the IETF language elements from the legacy elements. See #3557.
* AppImage: the AppImage is now built on AlmaLinux 8 with Qt 6.5.1. This means
  that the minimum glibc version this AppImage is compatible with has also
  been bumped, to v2.28. The oldest Debian coming with this version is Debian
  10 "Buster"; for Ubuntu it's 18.10 "Cosmic Cuttlefish".
* Windows installer: the installer will now remove the file
  `data\portable-app` if it's present in the installation directory as its
  presence would signal to the app to run in portable mode, requiring write
  access to its base directory. This fixes some installation issues that could
  occur when a user extracted a portable app archive into a folder like
  `C:\Program Files\MKVToolNix`. Also added an installation check for the same
  issue. See #3558.

## Bug fixes

* all: Linux: if initializing the locale system based on the system's settings
  fails, e.g. if environment variables such as `LC_ALL` have been set to
  `fr_FR.UTF-8` but the locale for `fr_FR.UTF-8` hasn't been built, MKVToolNix
  will try to fall back to `en_US.UTF-8` first & `C.UTF-8` second. If neither
  of the three succeeds, an error message will be shown, and the program will
  abort. This fixes the programs not catching an exception from the
  `boost::filesystem` library which doesn't cope with mis-configured locale
  systems well. Workaround for #3574.
* mkvmerge: sorting tracks by type did not work properly when some tracks
  weren't copied. Part of the fix of #3567.
* mkvmerge: track numbers in the destination file were assigned out-of-order
  when the track order in the source files differs from the one in the
  destination file. Now they're assigned sequentially in the order the tracks
  appear in the destination file again. Part of the fix of #3567.
* MKVToolNix GUI: under certain circumstances the GUI would try to display
  error messages from a secondary thread, which isn't allowed in Qt, causing
  to program to either hang or outright crash. Part of the fix of #3556 &
  #3561.
* MKVToolNix GUI: the GUI will check for several potential installation issues
  wrt. to having write access to certain directories (system's temporary
  directory, MKVToolNix's file identification cache directory and others) &
  display proper error messages if not. Part of the fix of #3556 & #3561.
* MKVToolNix GUI: header editor: fixed the application crashing whenever you
  change a property below the "segment information" node. This bug was
  introduced in v77. Fixes #3556.
* MKVToolNix GUI: header editor: when reading files that only contain legacy
  track language elements but no IETF track language elements, the header
  editor was showing all tracks as having "English" as the language. This bug
  was introduced in v77. Fixes #3557.
* MKVToolNix GUI: header editor: when a legacy track language element contains
  a country code (e.g. `fr-CA`), the header editor was wrongfully using `und`
  = "undetermined" as the language for that element. Now it'll use the
  language from the language/country pair, which would be `fr` in the earlier
  example. Part of the fix of #3557.
* MKVToolNix GUI: header editor: the keyboard shortcuts from the "modify
  selected track" sub-menu didn't work right after opening a file. They only
  started working after opening the "header editor" menu. Fixes #3573.
* AppImage: fixed the GUI resources (e.g. icons) not being found.
* AppImage: the Qt plugins required for using compose key/dead keys are now
  included. Fixes #3577.

## Build system changes

* The compilation database (`compile_commands.json`) is now always built. The
  variable `BUILD_COMPILATION_DATABASE` is not evaluated anymore.