please add en_DK locale to the built-in locale (mostly for sane iso 8601 date formats)

On linux Qt seems to be falling back to its built-in locale db. Unfortunately this db is missing an English locale with proper ISO date formatting (ISO8601). On most linux systems an "en_DK.UTF-8" locale is introduced to provide an English locale for folks living in Europe (or for others wishing to use an English locale with sane date and time formatting).

Background:
KDE has dropped its locale system and now relies on QLocale, so the missing en_DK locale is hurting a lot of users now (it's not clear if some or all of the customization features from the old KDE locale will ever come back). I'm asking for this minor addition to Qt mostly as a quick-fix for the KDE issue. Preferably the whole builtin locale db should be replaced with something better, but in the meantime this quick fix would help a lot of users.

See https://bugs.kde.org/show_bug.cgi?id=340982 for reference.