MKVToolNix 7.3.0

Update to latest stable release, add option to build experimental QT
GUI, add libmagic as recommended dependency, upgrade flac dependency to
recommended, downgrade gettext dependency to optional, remove
unnecessary configure options locating ogg and vorbis, change format of
`drake install` command.
mkvtoolnix: rename QT option

Rename preview-gui option to "--with-qt5", add description of wxWidgets
option.
mkvtoolnix: update 7.3.0 bottle.